diff --git a/docs/release_notes.rst b/docs/release_notes.rst
index 3c7b89fb..84f0b48c 100644
--- a/docs/release_notes.rst
+++ b/docs/release_notes.rst
@@ -1,6 +1,18 @@
 Release Notes
 =============
 
+v1.6.0
+------
+
+* Add warning about calling ``open()`` or ``copy()`` with filesystem paths that
+  are ambiguous. In the next version of stor, this will be an error, to prevent
+  differences between local and OBS code.  Set
+  ``stor:raise_on_ambiguous_filesystem_path`` to get this behavior now.
+* ``open()`` now raises with an invalid OBS path, rather than generating a
+  potentially harmful object.
+* Add ``stor.test.TestCase`` that allows mocking out of both swift AND s3
+  functionality in a test case.
+
 v1.5.4
 ------
 
diff --git a/stor/base.py b/stor/base.py
index bae3dca5..46c7f478 100644
--- a/stor/base.py
+++ b/stor/base.py
@@ -267,6 +267,8 @@ def open(self, *args, **kwargs):
         Args:
             mode (str): first positional arg, mode of file descriptor
             encoding (str): text encoding to use (Python 3 only)
+        Raises:
+            ValueError: if ambiguous path - see ``stor.utils.validate_file_path`` for more.
         """
 
         raise NotImplementedError
@@ -370,14 +372,17 @@ class FileSystemPath(Path):
     """
     def open(self, *args, **kwargs):
         """
-        Opens a path and retains interface compatibility with
-        `SwiftPath` by popping the unused ``swift_upload_args`` keyword
-        argument.
+        Opens a path.
 
         Creates parent directory if it does not exist.
+
+        Raises:
+            ValueError: if ambiguous path - see ``stor.utils.validate_file_path`` for more.
         """
-        kwargs.pop('swift_upload_kwargs', None)
+        if kwargs.pop('swift_upload_kwargs', None):
+            warnings.warn('swift_upload_kwargs will be removed in stor 2.0')
         self.parent.makedirs_p()
+        utils.validate_file_path(self)
         return builtins.open(self, *args, **kwargs)
 
     def __enter__(self):
diff --git a/stor/default.cfg b/stor/default.cfg
index 82ee1afd..3211d10c 100644
--- a/stor/default.cfg
+++ b/stor/default.cfg
@@ -1,5 +1,8 @@
 [stor]
 
+# set to raise if file paths are ambiguous - see stor.utils.validate_file_path for more.
+raise_on_ambiguous_filesystem_path = False
+
 [s3]
 
 # See boto3 docs for more detail on these parameters - all passed directly to boto3.session.Session *if* set
diff --git a/stor/obs.py b/stor/obs.py
index 5d79bb13..92a3d928 100644
--- a/stor/obs.py
+++ b/stor/obs.py
@@ -1,6 +1,7 @@
 import locale
 import posixpath
 import sys
+import warnings
 
 from cached_property import cached_property
 import six
@@ -90,6 +91,7 @@ def is_ambiguous(self):
         """Returns true if it cannot be determined if the path is a
         file or directory
         """
+        warnings.warn('this function is deprecated and will be removed in stor 2.0')
         return not self.endswith('/') and not self.ext
 
     def _get_parts(self):
@@ -282,11 +284,14 @@ def __init__(self, pth, mode='r', encoding=None, **kwargs):
                 use binary mode OR explicitly set an encoding when reading/writing text (because
                 writers from different computers may store data on OBS in different ways).
                 Python 3 only.
+        Raises:
+            ValueError: if the mode is not valid or the path is ambiguous (see ``stor.copy``)
         """
         if mode not in self._VALID_MODES:
             raise ValueError('invalid mode for file: %r' % mode)
         if six.PY2 and encoding:  # pragma: no cover
             raise TypeError('encoding not supported in Python 2')
+        utils.validate_file_path(pth)
         self._path = pth
         self.mode = mode
         self._kwargs = kwargs
diff --git a/stor/swift.py b/stor/swift.py
index 51c4aacd..a37b34b2 100644
--- a/stor/swift.py
+++ b/stor/swift.py
@@ -688,11 +688,7 @@ def open(self, mode='r', encoding=None, swift_upload_options=None):
                 ("r" or "rb") and writing ("w", "wb")
             encoding (str): text encoding to use. Defaults to
                 ``locale.getpreferredencoding(False)`` (Python 3 only)
-            swift_upload_options (dict): DEPRECATED (use `stor.settings.use()`
-                instead). A dictionary of arguments that will be
-                passed as keyword args to `SwiftPath.upload` if any writes
-                occur on the opened resource.
-
+            swift_upload_options (dict): DEPRECATED (use `stor.settings.use()` instead).
         Returns:
             SwiftFile: The swift object.
 
@@ -700,6 +696,8 @@ def open(self, mode='r', encoding=None, swift_upload_options=None):
             SwiftError: A swift client error occurred.
         """
         swift_upload_options = swift_upload_options or {}
+        if swift_upload_options:
+            warnings.warn('swift_upload_options will be removed in stor 2.0')
         return SwiftFile(self, mode=mode, encoding=encoding, **swift_upload_options)
 
     @_swift_retry(exceptions=(ConditionNotMetError, UnavailableError))
diff --git a/stor/test.py b/stor/test.py
index b5c31dd5..3f8b5b0d 100644
--- a/stor/test.py
+++ b/stor/test.py
@@ -207,3 +207,7 @@ def setUp(self):
             del s3._thread_local.s3_transfer_config
         except AttributeError:
             pass
+
+
+class StorTestCase(S3TestCase, SwiftTestCase):
+    """A TestCase class that mocks BOTH s3 and stor for tests"""
diff --git a/stor/tests/test_cli.py b/stor/tests/test_cli.py
index 9228f7c7..891ed5f5 100644
--- a/stor/tests/test_cli.py
+++ b/stor/tests/test_cli.py
@@ -40,7 +40,9 @@ def test_cli_error(self, mock_list, mock_stderr):
     @mock.patch('stor.settings.USER_CONFIG_FILE', '')
     def test_cli_config(self, mock_copytree):
         expected_settings = {
-            'stor': {},
+            'stor': {
+                'raise_on_ambiguous_filesystem_path': False
+            },
             's3': {
                 'aws_access_key_id': '',
                 'aws_secret_access_key': '',
@@ -444,14 +446,14 @@ def test_walkfiles_no_pattern(self, mock_walkfiles):
             './a/b.txt',
             './c.txt',
             './d.txt',
-            './file'
+            './file.ext'
         ]
         self.parse_args('stor walkfiles .')
         self.assertEquals(sys.stdout.getvalue(),
                           './a/b.txt\n'
                           './c.txt\n'
                           './d.txt\n'
-                          './file\n')
+                          './file.ext\n')
         mock_walkfiles.assert_called_once_with(PosixPath('.'))
 
 
@@ -459,16 +461,16 @@ class TestCat(BaseCliTest):
     @mock.patch.object(S3Path, 'read_object', autospec=True)
     def test_cat_s3(self, mock_read):
         mock_read.return_value = b'hello world\n'
-        self.parse_args('stor cat s3://test/file')
+        self.parse_args('stor cat s3://test/file.ext')
         self.assertEquals(sys.stdout.getvalue(), 'hello world\n')
-        mock_read.assert_called_once_with(S3Path('s3://test/file'))
+        mock_read.assert_called_once_with(S3Path('s3://test/file.ext'))
 
     @mock.patch.object(SwiftPath, 'read_object', autospec=True)
     def test_cat_swift(self, mock_read):
         mock_read.return_value = b'hello world'
-        self.parse_args('stor cat swift://some/test/file')
+        self.parse_args('stor cat swift://some/test/file.ext')
         self.assertEquals(sys.stdout.getvalue(), 'hello world\n')
-        mock_read.assert_called_once_with(SwiftPath('swift://some/test/file'))
+        mock_read.assert_called_once_with(SwiftPath('swift://some/test/file.ext'))
 
 
 class TestCd(BaseCliTest):
@@ -507,14 +509,14 @@ def test_cd_swift(self, mock_isdir):
     @mock.patch('sys.stderr', new=six.StringIO())
     def test_cd_not_dir_s3(self, mock_isdir):
         with self.assertRaisesRegexp(SystemExit, '1'):
-            self.parse_args('stor cd s3://test/file')
+            self.parse_args('stor cd s3://test/file.ext')
         self.assertIn('not a directory', sys.stderr.getvalue())
 
     @mock.patch.object(SwiftPath, 'isdir', return_value=False, autospec=True)
     @mock.patch('sys.stderr', new=six.StringIO())
     def test_cd_not_dir_swift(self, mock_isdir):
         with self.assertRaisesRegexp(SystemExit, '1'):
-            self.parse_args('stor cd swift://test/container/file')
+            self.parse_args('stor cd swift://test/container/file.ext')
         self.assertIn('not a directory', sys.stderr.getvalue())
 
     @mock.patch('sys.stderr', new=six.StringIO())
diff --git a/stor/tests/test_posix.py b/stor/tests/test_posix.py
index fca2a28c..d7b4033c 100644
--- a/stor/tests/test_posix.py
+++ b/stor/tests/test_posix.py
@@ -92,25 +92,6 @@ def test_posix_file_destination(self):
             self.assertTrue(dest.exists())
             self.assertEquals(dest.open().read(), '1')
 
-    def test_ambigious_swift_resource_destination(self):
-        with stor.NamedTemporaryDirectory() as tmp_d:
-            source = tmp_d / '1'
-            with open(source, 'w') as tmp_file:
-                tmp_file.write('1')
-
-            dest = 'swift://tenant/container/ambiguous-resource'
-            with self.assertRaisesRegexp(ValueError, 'OBS destination'):
-                utils.copy(source, dest)
-
-    def test_ambigious_swift_container_destination(self):
-        with stor.NamedTemporaryDirectory() as tmp_d:
-            source = tmp_d / '1'
-            with open(source, 'w') as tmp_file:
-                tmp_file.write('1')
-
-            dest = 'swift://tenant/ambiguous-container'
-            with self.assertRaisesRegexp(ValueError, 'OBS destination'):
-                utils.copy(source, dest)
 
     def test_tenant_swift_destination(self):
         with stor.NamedTemporaryDirectory() as tmp_d:
@@ -119,7 +100,7 @@ def test_tenant_swift_destination(self):
             with open(source / '1', 'w') as tmp_file:
                 tmp_file.write('1')
 
-            dest = 'swift://tenant/'
+            dest = 'swift://tenant.with.dots'
             with self.assertRaisesRegexp(ValueError, 'copy to tenant'):
                 utils.copy(source / '1', dest)
 
diff --git a/stor/tests/test_s3.py b/stor/tests/test_s3.py
index 59cf303c..d28c1642 100644
--- a/stor/tests/test_s3.py
+++ b/stor/tests/test_s3.py
@@ -101,16 +101,16 @@ def test_resource_none_w_bucket(self):
         self.assertIsNone(s3_p.resource)
 
     def test_resource_object(self):
-        s3_p = S3Path('s3://bucket/obj')
-        self.assertEquals(s3_p.resource, 'obj')
+        s3_p = S3Path('s3://bucket/obj.ext')
+        self.assertEquals(s3_p.resource, 'obj.ext')
 
     def test_resource_single_dir(self):
         s3_p = S3Path('s3://bucket/dir/')
         self.assertEquals(s3_p.resource, 'dir/')
 
     def test_resource_nested_obj(self):
-        s3_p = S3Path('s3://bucket/nested/obj')
-        self.assertEquals(s3_p.resource, 'nested/obj')
+        s3_p = S3Path('s3://bucket/nested/obj.ext')
+        self.assertEquals(s3_p.resource, 'nested/obj.ext')
 
     def test_resource_nested_dir(self):
         s3_p = S3Path('s3://bucket/nested/dir/')
@@ -348,14 +348,14 @@ def test_list_condition(self):
 
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_list_w_use_manifest(self, mock_stream):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list = self.mock_s3_iterator
         mock_list.__iter__.return_value = [{
             'Contents': [
-                {'Key': 'my/obj1'},
-                {'Key': 'my/obj2'},
-                {'Key': 'my/obj3'}
+                {'Key': 'my/obj1.ext'},
+                {'Key': 'my/obj2.ext'},
+                {'Key': 'my/obj3.ext'}
             ],
             'IsTruncated': False
         }]
@@ -363,20 +363,20 @@ def test_list_w_use_manifest(self, mock_stream):
         s3_p = S3Path('s3://bucket')
         results = s3_p.list(use_manifest=True)
         self.assertEquals(set(results), set([
-            's3://bucket/my/obj1',
-            's3://bucket/my/obj2',
-            's3://bucket/my/obj3'
+            's3://bucket/my/obj1.ext',
+            's3://bucket/my/obj2.ext',
+            's3://bucket/my/obj3.ext'
         ]))
 
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_list_w_use_manifest_validation_err(self, mock_stream):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list = self.mock_s3_iterator
         mock_list.__iter__.return_value = [{
             'Contents': [
-                {'Key': 'my/obj1'},
-                {'Key': 'my/obj2'}
+                {'Key': 'my/obj1.ext'},
+                {'Key': 'my/obj2.ext'}
             ],
             'IsTruncated': False
         }]
@@ -387,14 +387,14 @@ def test_list_w_use_manifest_validation_err(self, mock_stream):
 
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_list_w_condition_and_use_manifest(self, mock_stream):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list = self.mock_s3_iterator
         mock_list.__iter__.return_value = [{
             'Contents': [
-                {'Key': 'my/obj1'},
-                {'Key': 'my/obj2'},
-                {'Key': 'my/obj3'}
+                {'Key': 'my/obj1.ext'},
+                {'Key': 'my/obj2.ext'},
+                {'Key': 'my/obj3.ext'}
             ],
             'IsTruncated': False
         }]
@@ -402,9 +402,9 @@ def test_list_w_condition_and_use_manifest(self, mock_stream):
         s3_p = S3Path('s3://bucket')
         results = s3_p.list(use_manifest=True, condition=lambda results: len(results) == 3)
         self.assertEquals(set(results), set([
-            's3://bucket/my/obj1',
-            's3://bucket/my/obj2',
-            's3://bucket/my/obj3'
+            's3://bucket/my/obj1.ext',
+            's3://bucket/my/obj2.ext',
+            's3://bucket/my/obj3.ext'
         ]))
 
     def test_list_ignore_dir_markers(self):
@@ -641,7 +641,7 @@ def test_getsize_file(self):
             'ContentLength': 15,
             'ContentType': 'text/plain'
         }
-        s3_p = S3Path('s3://bucket/obj')
+        s3_p = S3Path('s3://bucket/obj.ext')
         self.assertEquals(s3_p.getsize(), 15)
 
     @mock.patch.object(S3Path, 'exists', autospec=True)
@@ -834,17 +834,17 @@ def test_rmtree_dir(self, mock_list):
     def test_rmtree_over_1000(self, mock_list):
         mock_delete_objects = self.mock_s3.delete_objects
         mock_delete_objects.return_value = {}
-        objects = [S3Path('s3://bucket/obj' + str(i)) for i in range(1234)]
+        objects = [S3Path('s3://bucket/obj.ext' + str(i)) for i in range(1234)]
         mock_list.return_value = objects
 
         s3_p = S3Path('s3://bucket')
         s3_p.rmtree()
         mock_delete_objects.assert_has_calls([
             mock.call(Bucket='bucket', Delete={
-                'Objects': [{'Key': 'obj' + str(i)} for i in range(1000)]
+                'Objects': [{'Key': 'obj.ext' + str(i)} for i in range(1000)]
             }),
             mock.call(Bucket='bucket', Delete={
-                'Objects': [{'Key': 'obj' + str(i + 1000)} for i in range(234)]
+                'Objects': [{'Key': 'obj.ext' + str(i + 1000)} for i in range(234)]
             })
         ])
         self.assertEquals([], objects)
@@ -1228,12 +1228,12 @@ def test_download_w_condition(self, mock_list, mock_getsize, mock_make_dest):
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_download_w_use_manifest(self, mock_stream, mock_list, mock_getsize,
                                      mock_make_dest_dir):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2'),
-            S3Path('s3://bucket/my/obj3')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext'),
+            S3Path('s3://bucket/my/obj3.ext')
         ]
 
         s3_p = S3Path('s3://bucket')
@@ -1244,11 +1244,11 @@ def test_download_w_use_manifest(self, mock_stream, mock_list, mock_getsize,
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_download_w_use_manifest_validation_err(self, mock_stream, mock_list, mock_getsize,
                                                     mock_make_dest_dir):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext')
         ]
 
         s3_p = S3Path('s3://bucket')
@@ -1259,12 +1259,12 @@ def test_download_w_use_manifest_validation_err(self, mock_stream, mock_list, mo
     @mock.patch('botocore.response.StreamingBody', autospec=True)
     def test_download_w_condition_and_use_manifest(self, mock_stream, mock_list, mock_getsize,
                                                    mock_make_dest_dir):
-        mock_stream.read.return_value = b'my/obj1\nmy/obj2\nmy/obj3\n'
+        mock_stream.read.return_value = b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2'),
-            S3Path('s3://bucket/my/obj3')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext'),
+            S3Path('s3://bucket/my/obj3.ext')
         ]
 
         s3_p = S3Path('s3://bucket')
@@ -1290,9 +1290,9 @@ def test_download_object_threads(self, mock_pool, mock_list, mock_getsize,
     @mock.patch.object(S3Path, 'list', autospec=True)
     def test_download_remote_error(self, mock_list, mock_getsize, mock_make_dest_dir):
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2'),
-            S3Path('s3://bucket/my/obj3')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext'),
+            S3Path('s3://bucket/my/obj3.ext')
         ]
         self.mock_s3_transfer.download_file.side_effect = RetriesExceededError('failed')
 
@@ -1302,9 +1302,9 @@ def test_download_remote_error(self, mock_list, mock_getsize, mock_make_dest_dir
     @mock.patch.object(S3Path, 'list', autospec=True)
     def test_download_other_error(self, mock_list, mock_getsize, mock_make_dest_dir):
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2'),
-            S3Path('s3://bucket/my/obj3')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext'),
+            S3Path('s3://bucket/my/obj3.ext')
         ]
         self.mock_s3_transfer.download_file.side_effect = [None, ValueError]
 
@@ -1314,9 +1314,9 @@ def test_download_other_error(self, mock_list, mock_getsize, mock_make_dest_dir)
     @mock.patch.object(S3Path, 'list', autospec=True)
     def test_download_multipart_settings(self, mock_list, mock_getsize, mock_make_dest_dir):
         mock_list.return_value = [
-            S3Path('s3://bucket/my/obj1'),
-            S3Path('s3://bucket/my/obj2'),
-            S3Path('s3://bucket/my/obj3')
+            S3Path('s3://bucket/my/obj1.ext'),
+            S3Path('s3://bucket/my/obj2.ext'),
+            S3Path('s3://bucket/my/obj3.ext')
         ]
         s3_p = S3Path('s3://bucket')
         with settings.use({'s3:download': {'segment_size': '5M', 'segment_threads': 20}}):
@@ -1360,14 +1360,14 @@ def test_copy_posix_dir_destination(self, mockdownload_object):
             mockdownload_object.assert_called_once_with(p, Path(tmp_d) / 'file_source.txt')
 
     def test_copy_swift_destination(self):
-        p = S3Path('s3://bucket/key/file_source')
+        p = S3Path('s3://bucket/key/file_source.ext')
         with self.assertRaisesRegexp(ValueError, 'OBS path'):
-            p.copy('swift://tenant/container/file_dest')
+            p.copy('swift://tenant/container/file_dest.ext')
 
     def test_copy_s3_destination(self):
-        p = S3Path('s3://bucket/key/file_source')
+        p = S3Path('s3://bucket/key/file_source.ext')
         with self.assertRaisesRegexp(ValueError, 'OBS path'):
-            p.copy('s3://bucket/key/file_dest')
+            p.copy('s3://bucket/key/file_dest.ext')
 
 
 class TestCopytree(S3TestCase):
@@ -1395,7 +1395,7 @@ def test_copytree_windows_destination(self):
 
 class TestS3File(S3TestCase):
     def test_invalid_buffer_mode(self):
-        s3_f = S3Path('s3://bucket/key/obj').open()
+        s3_f = S3Path('s3://bucket/key/obj.ext').open()
         s3_f.mode = 'invalid'
         with self.assertRaisesRegexp(ValueError, 'buffer'):
             s3_f._buffer
@@ -1404,13 +1404,13 @@ def test_invalid_buffer_mode(self):
     def test_invalid_flush_mode(self, mock_stream):
         mock_stream.read.return_value = b'data'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open()
         with self.assertRaisesRegexp(TypeError, 'flush'):
             obj.flush()
 
     def test_name(self):
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open()
         self.assertEquals(obj.name, s3_p)
 
@@ -1418,7 +1418,7 @@ def test_name(self):
     def test_context_manager_on_closed_file(self, mock_stream):
         mock_stream.read.return_value = b'data'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open()
         obj.close()
 
@@ -1427,7 +1427,7 @@ def test_context_manager_on_closed_file(self, mock_stream):
                 pass  # pragma: no cover
 
     def test_invalid_mode(self):
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         with self.assertRaisesRegexp(ValueError, 'invalid mode'):
             s3_p.open(mode='invalid')
 
@@ -1443,7 +1443,7 @@ class MyFile(object):
     def test_read_on_closed_file(self, mock_stream):
         mock_stream.read.return_value = b'data'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open()
         obj.close()
 
@@ -1451,7 +1451,7 @@ def test_read_on_closed_file(self, mock_stream):
             obj.read()
 
     def test_read_invalid_mode(self):
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         with self.assertRaisesRegexp(TypeError, 'mode.*read'):
             s3_p.open(mode='wb').read()
 
@@ -1460,7 +1460,7 @@ def test_read_success(self, mock_stream):
         mock_stream.read.return_value = b'data'
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
 
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         self.assertEquals(s3_p.open().read(), 'data')
 
     @mock.patch('botocore.response.StreamingBody', autospec=True)
@@ -1474,7 +1474,7 @@ def test_iterating_over_files(self, mock_stream):
         mock_stream.read.return_value = data
         self.mock_s3.get_object.return_value = {'Body': mock_stream}
 
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         self.assertEquals(s3_p.open().read(), data.decode('ascii'))
         self.assertEquals(s3_p.open().readlines(),
                           [l + '\n' for l in data.decode('ascii').split('\n')][:-1])
@@ -1485,7 +1485,7 @@ def test_iterating_over_files(self, mock_stream):
         self.assertEqual(next(iter(s3_p.open())), 'line1\n')
 
     def test_write_invalid_args(self):
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open(mode='r')
         with self.assertRaisesRegexp(TypeError, 'mode.*write'):
             obj.write('hello')
@@ -1493,7 +1493,7 @@ def test_write_invalid_args(self):
     @mock.patch('time.sleep', autospec=True)
     def test_binary_write_multiple_w_context_manager(self, mock_sleep):
         mock_upload = self.mock_s3_transfer.upload_file
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         with s3_p.open(mode='wb') as obj:
             obj.write(b'hello')
             obj.write(b' world')
@@ -1504,7 +1504,7 @@ def test_binary_write_multiple_w_context_manager(self, mock_sleep):
     @mock.patch('time.sleep', autospec=True)
     def test_write_multiple_flush_multiple_upload(self, mock_sleep):
         mock_upload = self.mock_s3_transfer.upload_file
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         with NamedTemporaryFile('wb', delete=False) as ntf1,\
                 NamedTemporaryFile('wb', delete=False) as ntf2,\
                 NamedTemporaryFile('wb', delete=False) as ntf3:
@@ -1534,7 +1534,7 @@ def test_write_multiple_flush_multiple_upload(self, mock_sleep):
     @mock.patch('time.sleep', autospec=True)
     def test_close_no_writes(self, mock_sleep):
         mock_upload = self.mock_s3_transfer.upload_file
-        s3_p = S3Path('s3://bucket/key/obj')
+        s3_p = S3Path('s3://bucket/key/obj.ext')
         obj = s3_p.open(mode='wb')
         obj.close()
 
diff --git a/stor/tests/test_settings.py b/stor/tests/test_settings.py
index 74f57e6e..df32f262 100644
--- a/stor/tests/test_settings.py
+++ b/stor/tests/test_settings.py
@@ -32,7 +32,9 @@ class TestSettings(unittest.TestCase):
     @mock.patch.dict(os.environ, {}, clear=True)
     def test_initialize_default(self):
         expected_settings = {
-            'stor': {},
+            'stor': {
+                'raise_on_ambiguous_filesystem_path': False,
+            },
             's3': {
                 'aws_access_key_id': '',
                 'aws_secret_access_key': '',
@@ -83,7 +85,9 @@ def test_initialize_default(self):
     @mock.patch.dict(os.environ, {}, clear=True)
     def test_initialize_w_user_file(self):
         expected_settings = {
-            'stor': {},
+            'stor': {
+                'raise_on_ambiguous_filesystem_path': False,
+            },
             's3': {
                 'aws_access_key_id': '',
                 'aws_secret_access_key': '',
diff --git a/stor/tests/test_swift.py b/stor/tests/test_swift.py
index bfc38d10..11a9d440 100644
--- a/stor/tests/test_swift.py
+++ b/stor/tests/test_swift.py
@@ -87,7 +87,7 @@ def test_basename(self):
 class TestManifestUtilities(unittest.TestCase):
     def test_generate_save_and_read_manifest(self):
         with NamedTemporaryDirectory() as tmp_d:
-            manifest_contents = ['obj1', 'dir/obj2']
+            manifest_contents = ['obj1', 'dir/obj2.ext']
             utils.generate_and_save_data_manifest(tmp_d, manifest_contents)
             read_contents = utils.get_data_manifest_contents(tmp_d)
 
@@ -162,8 +162,8 @@ def test_w_container_no_resource(self):
         self.assertIsNone(swift_p.resource)
 
     def test_resource_single_object(self):
-        swift_p = SwiftPath('swift://tenant/container/obj')
-        self.assertEquals(swift_p.resource, 'obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
+        self.assertEquals(swift_p.resource, 'obj.ext')
 
     def test_resource_single_dir_w_slash(self):
         swift_p = SwiftPath('swift://tenant/container/dir/')
@@ -233,29 +233,29 @@ def setUp(self):
 
     def test_makedirs_p_does_nothing(self):
         # dumb test... but why not?
-        SwiftPath('swift://tenant/container/obj').makedirs_p()
+        SwiftPath('swift://tenant/container/obj.ext').makedirs_p()
 
     def test_invalid_buffer_mode(self):
-        swift_f = SwiftPath('swift://tenant/container/obj').open()
+        swift_f = SwiftPath('swift://tenant/container/obj.ext').open()
         swift_f.mode = 'invalid'
         with self.assertRaisesRegexp(ValueError, 'buffer'):
             swift_f._buffer
 
     def test_invalid_flush_mode(self):
         self.mock_swift_conn.get_object.return_value = ('header', b'data')
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open()
         with self.assertRaisesRegexp(TypeError, 'flush'):
             obj.flush()
 
     def test_name(self):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open()
         self.assertEquals(obj.name, swift_p)
 
     def test_context_manager_on_closed_file(self):
         self.mock_swift_conn.get_object.return_value = ('header', b'data')
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open()
         obj.close()
 
@@ -264,7 +264,7 @@ def test_context_manager_on_closed_file(self):
                 pass  # pragma: no cover
 
     def test_invalid_mode(self):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         with self.assertRaisesRegexp(ValueError, 'invalid mode'):
             swift_p.open(mode='invalid')
 
@@ -278,7 +278,7 @@ class MyFile(object):
 
     def test_read_on_closed_file(self):
         self.mock_swift_conn.get_object.return_value = ('header', b'data')
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open()
         obj.close()
 
@@ -286,14 +286,14 @@ def test_read_on_closed_file(self):
             obj.read()
 
     def test_read_invalid_mode(self):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         with self.assertRaisesRegexp(TypeError, 'mode.*read'):
             swift_p.open(mode='wb').read()
 
     def test_read_success(self):
         self.mock_swift_conn.get_object.return_value = ('header', b'data')
 
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         self.assertEquals(swift_p.open().read(), 'data')
 
     def test_iterating_over_files(self):
@@ -305,7 +305,7 @@ def test_iterating_over_files(self):
 '''
         self.mock_swift_conn.get_object.return_value = ('header', data)
 
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         # open().read() should return str for r
         self.assertEquals(swift_p.open('r').read(), data.decode('ascii'))
         # open().read() should return bytes for rb
@@ -324,13 +324,13 @@ def test_read_success_on_second_try(self, mock_sleep):
             ClientException('dummy', 'dummy', http_status=404),
             ('header', b'data')
         ]
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open()
         self.assertEquals(obj.read(), 'data')
         self.assertEquals(len(mock_sleep.call_args_list), 1)
 
     def test_write_invalid_args(self):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open(mode='r')
         with self.assertRaisesRegexp(TypeError, 'mode.*write'):
             obj.write('hello')
@@ -342,7 +342,7 @@ def test_write_use_manifest_multiple_and_close(self, mock_upload, mock_sleep):
             with mock.patch('tempfile.NamedTemporaryFile',
                             autospec=True) as ntf_mock:
                 ntf_mock.side_effect = [fp]
-                swift_p = SwiftPath('swift://tenant/container/obj')
+                swift_p = SwiftPath('swift://tenant/container/obj.ext')
                 obj = swift_p.open(mode='wb', swift_upload_options={
                     'use_manifest': True
                 })
@@ -358,7 +358,7 @@ def test_write_use_manifest_multiple_and_close(self, mock_upload, mock_sleep):
     @mock.patch('time.sleep', autospec=True)
     @mock.patch.object(SwiftPath, 'upload', autospec=True)
     def test_write_multiple_w_context_manager(self, mock_upload, mock_sleep):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         with swift_p.open(mode='wb') as obj:
             obj.write(b'hello')
             obj.write(b' world')
@@ -368,7 +368,7 @@ def test_write_multiple_w_context_manager(self, mock_upload, mock_sleep):
     @mock.patch.object(SwiftPath, 'upload', autospec=True)
     def test_write_multiple_flush_multiple_upload(self, mock_upload,
                                                   mock_sleep):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         with NamedTemporaryFile(delete=False) as ntf1,\
                 NamedTemporaryFile(delete=False) as ntf2,\
                 NamedTemporaryFile(delete=False) as ntf3:
@@ -395,7 +395,7 @@ def test_write_multiple_flush_multiple_upload(self, mock_upload,
     @mock.patch('time.sleep', autospec=True)
     @mock.patch.object(SwiftPath, 'upload', autospec=True)
     def test_close_no_writes(self, mock_upload, mock_sleep):
-        swift_p = SwiftPath('swift://tenant/container/obj')
+        swift_p = SwiftPath('swift://tenant/container/obj.ext')
         obj = swift_p.open(mode='wb')
         obj.close()
 
@@ -484,7 +484,7 @@ def test_no_auth_url(self):
         }
         with settings.use(temp_settings):
             with self.assertRaisesRegexp(ValueError, 'auth url'):
-                SwiftPath('swift://tenant/container/obj').temp_url()
+                SwiftPath('swift://tenant/container/obj.ext').temp_url()
 
     def test_no_temp_url_key(self):
         temp_settings = {
@@ -495,7 +495,7 @@ def test_no_temp_url_key(self):
         }
         with settings.use(temp_settings):
             with self.assertRaisesRegexp(ValueError, 'temporary url key'):
-                SwiftPath('swift://tenant/container/obj').temp_url()
+                SwiftPath('swift://tenant/container/obj.ext').temp_url()
 
 
 class TestList(SwiftTestCase):
@@ -892,52 +892,52 @@ def test_list_starts_with_no_resource(self):
 
     @mock.patch('time.sleep', autospec=True)
     def test_list_w_condition_and_use_manifest(self, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         mock_list = self.mock_swift_conn.get_container
         mock_list.return_value = ({}, [{
-            'name': 'my/obj1'
+            'name': 'my/obj1.ext'
         }, {
-            'name': 'my/obj2'
+            'name': 'my/obj2.ext'
         }, {
-            'name': 'my/obj3'
+            'name': 'my/obj3.ext'
         }])
 
         swift_p = SwiftPath('swift://tenant/container/')
         results = swift_p.list(use_manifest=True, condition=lambda results: len(results) == 3)
         self.assertEquals(set(results), set([
-            'swift://tenant/container/my/obj1',
-            'swift://tenant/container/my/obj2',
-            'swift://tenant/container/my/obj3'
+            'swift://tenant/container/my/obj1.ext',
+            'swift://tenant/container/my/obj2.ext',
+            'swift://tenant/container/my/obj3.ext'
         ]))
 
     @mock.patch('time.sleep', autospec=True)
     def test_list_use_manifest(self, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         mock_list = self.mock_swift_conn.get_container
         mock_list.return_value = ({}, [{
-            'name': 'my/obj1'
+            'name': 'my/obj1.ext'
         }, {
-            'name': 'my/obj2'
+            'name': 'my/obj2.ext'
         }, {
-            'name': 'my/obj3'
+            'name': 'my/obj3.ext'
         }])
 
         swift_p = SwiftPath('swift://tenant/container/')
         results = swift_p.list(use_manifest=True)
         self.assertEquals(set(results), set([
-            'swift://tenant/container/my/obj1',
-            'swift://tenant/container/my/obj2',
-            'swift://tenant/container/my/obj3'
+            'swift://tenant/container/my/obj1.ext',
+            'swift://tenant/container/my/obj2.ext',
+            'swift://tenant/container/my/obj3.ext'
         ]))
 
     @mock.patch('time.sleep', autospec=True)
     def test_list_use_manifest_validation_err(self, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         mock_list = self.mock_swift_conn.get_container
         mock_list.return_value = ({}, [{
-            'name': 'my/obj1'
+            'name': 'my/obj1.ext'
         }, {
-            'name': 'my/obj2'
+            'name': 'my/obj2.ext'
         }])
 
         swift_p = SwiftPath('swift://tenant/container/')
@@ -949,35 +949,35 @@ class TestWalkFiles(SwiftTestCase):
     def test_no_pattern_w_dir_markers(self):
         mock_list = self.mock_swift_conn.get_container
         mock_list.return_value = ({}, [{
-            'name': 'my/obj1',
+            'name': 'my/obj1.ext',
             'content_type': 'application/directory'
         }, {
-            'name': 'my/obj2',
+            'name': 'my/obj2.ext',
             'content_type': 'text/directory'
         }, {
-            'name': 'my/obj3',
+            'name': 'my/obj3.ext',
             'content_type': 'application/octet-stream'
         }, {
-            'name': 'my/obj4',
+            'name': 'my/obj4.ext',
             'content_type': 'application/octet-stream'
         }])
 
         f = list(SwiftPath('swift://tenant/container').walkfiles())
         self.assertEquals(set(f), set([
-            SwiftPath('swift://tenant/container/my/obj3'),
-            SwiftPath('swift://tenant/container/my/obj4')
+            SwiftPath('swift://tenant/container/my/obj3.ext'),
+            SwiftPath('swift://tenant/container/my/obj4.ext')
         ]))
 
     def test_w_pattern_w_dir_markers(self):
         mock_list = self.mock_swift_conn.get_container
         mock_list.return_value = ({}, [{
-            'name': 'my/obj1',
+            'name': 'my/obj1.ext',
             'content_type': 'application/directory'
         }, {
-            'name': 'my/obj2',
+            'name': 'my/obj2.ext',
             'content_type': 'text/directory'
         }, {
-            'name': 'my/obj3',
+            'name': 'my/obj3.ext',
             'content_type': 'application/octet-stream'
         }, {
             'name': 'my/obj4.sh',
@@ -986,7 +986,7 @@ def test_w_pattern_w_dir_markers(self):
             'name': 'my/other/obj5.sh',
             'content_type': 'application/octet-stream'
         }, {
-            'name': 'my/dirwithpattern.sh/obj6',
+            'name': 'my/dirwithpattern.sh/obj6.ext',
             'content_type': 'application/octet-stream'
         }])
 
@@ -1413,18 +1413,18 @@ def test_download_correct_thread_options(self):
     @mock.patch('time.sleep', autospec=True)
     @mock.patch.object(SwiftPath, 'list', autospec=True)
     def test_download_w_condition_and_use_manifest(self, mock_list, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         self.mock_swift.download.return_value = [{
             'action': 'download_object',
-            'object': 'my/obj1',
+            'object': 'my/obj1.ext',
             'success': True
         }, {
             'action': 'download_object',
-            'object': 'my/obj2',
+            'object': 'my/obj2.ext',
             'success': True
         }, {
             'action': 'download_object',
-            'object': 'my/obj3',
+            'object': 'my/obj3.ext',
             'success': True
         }]
 
@@ -1448,18 +1448,18 @@ def test_download_w_condition_and_use_manifest(self, mock_list, mock_sleep):
     @mock.patch('time.sleep', autospec=True)
     @mock.patch.object(SwiftPath, 'list', autospec=True)
     def test_download_w_use_manifest(self, mock_list, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         self.mock_swift.download.return_value = [{
             'action': 'download_object',
-            'object': 'my/obj1',
+            'object': 'my/obj1.ext',
             'success': True
         }, {
             'action': 'download_object',
-            'object': 'my/obj2',
+            'object': 'my/obj2.ext',
             'success': True
         }, {
             'action': 'download_object',
-            'object': 'my/obj3',
+            'object': 'my/obj3.ext',
             'success': True
         }]
 
@@ -1481,14 +1481,16 @@ def test_download_w_use_manifest(self, mock_list, mock_sleep):
     @mock.patch('time.sleep', autospec=True)
     @mock.patch.object(SwiftPath, 'list', autospec=True)
     def test_download_w_use_manifest_validation_err(self, mock_list, mock_sleep):
-        self.mock_swift_conn.get_object.return_value = ('header', b'my/obj1\nmy/obj2\nmy/obj3\n')
+        self.mock_swift_conn.get_object.return_value = (
+                'header',
+                b'my/obj1.ext\nmy/obj2.ext\nmy/obj3.ext\n')
         self.mock_swift.download.return_value = [{
             'action': 'download_object',
-            'object': 'my/obj1',
+            'object': 'my/obj1.ext',
             'success': True
         }, {
             'action': 'download_object',
-            'object': 'my/obj2',
+            'object': 'my/obj2.ext',
             'success': True
         }]
 
@@ -2021,9 +2023,9 @@ def test_copy_posix_dir_destination(self, mockdownload_object):
             mockdownload_object.assert_called_once_with(p, Path(tmp_d) / 'file_source.txt')
 
     def test_copy_swift_destination(self):
-        p = SwiftPath('swift://tenant/container/file_source')
+        p = SwiftPath('swift://tenant/container/file_source.txt')
         with self.assertRaisesRegexp(ValueError, 'OBS path'):
-            p.copy('swift://tenant/container/file_dest')
+            p.copy('swift://tenant/container/file_dest.txt')
 
 
 class TestCopytree(SwiftTestCase):
diff --git a/stor/tests/test_utils.py b/stor/tests/test_utils.py
index 1d1b7766..7c70ae18 100644
--- a/stor/tests/test_utils.py
+++ b/stor/tests/test_utils.py
@@ -1,12 +1,14 @@
 import errno
 import logging
-import mock
 import ntpath
 import os
 import stat
 import unittest
 
+
 from testfixtures import LogCapture
+import mock
+import pytest
 
 import stor
 from stor import Path
@@ -15,6 +17,7 @@
 from stor.swift import SwiftPath
 from stor.windows import WindowsPath
 from stor import utils
+from stor.test import StorTestCase
 
 
 class TestBaseProgressLogger(unittest.TestCase):
@@ -112,7 +115,7 @@ def test_w_broken_symlink(self):
         )
         with utils.NamedTemporaryDirectory(dir=swift_dir) as tmp_dir:
             symlink = tmp_dir / 'broken.symlink'
-            symlink_source = tmp_dir / 'nonexistent'
+            symlink_source = tmp_dir / 'nonexistent.txt'
             # put something in symlink source so that Python doesn't complain
             with stor.open(symlink_source, 'w') as fp:
                 fp.write('blah')
@@ -460,3 +463,47 @@ def test_path_no_perms(self):
         self.mock_copy.side_effect = stor.exceptions.FailedUploadError('foo')
         self.assertFalse(utils.is_writeable('s3://stor-test/foo/bar'))
         self.assertFalse(self.mock_remove.called)
+
+
+@mock.patch.object(stor.base.FileSystemPath, 'makedirs', lambda *args, **kwargs: None)
+class TestAmbiguityValidation(StorTestCase):
+    # someday we'll be able to use pytest.parametrized, but not today
+    DRIVES = ["swift://", "s3://", "/"]
+    def test_open_detects_invalid_modes(self):
+        def tester(drive):
+            with self.assertRaisesRegexp(ValueError, 'mode'):
+                with stor.open(stor.join(drive, 'dummy/something/path.txt'), 'invalid'):
+                    assert False, 'I should not be able to get into this block'
+        for drive in self.DRIVES:
+            tester(drive)
+
+    @mock.patch('stor.base.builtins.open', lambda *args, **kwargs: None)
+    def test_open_should_detect_invalid_paths(self):
+        def tester(drive, subpath):
+            with self.assertRaisesRegexp(ValueError, 'file paths'):
+                with stor.open(stor.join(drive, subpath), 'w'):
+                    assert False, 'I should not be able to get into this block'
+            with self.assertRaisesRegexp(ValueError, 'file paths'):
+                # prevent from going out of scope
+                f = stor.open(stor.join(drive, subpath))
+                assert False and not f
+            with self.assertRaisesRegexp(ValueError, 'file paths'):
+                utils.copy('whatever', stor.join(drive, subpath))
+        with stor.settings.use({'stor': {'raise_on_ambiguous_filesystem_path': True}}):
+            for drive in self.DRIVES:
+                for subpath in [
+                        'tenant/ambiguous-container', 'T/C/trailing-slash/', 'T/C/no-extension']:
+                    tester(drive, subpath)
+        with pytest.warns(UserWarning):
+            stor.open('/a/')
+
+    @mock.patch('stor.base.builtins.open', lambda *args, **kwargs: None)
+    def test_validate_path(self):
+        def tester(subpath, regex):
+            with stor.settings.use({'stor': {'raise_on_ambiguous_filesystem_path': True}}):
+                for drive in ['swift://', 's3://', '/']:
+                    with self.assertRaisesRegexp(ValueError, regex):
+                        utils.validate_file_path(stor.join(drive, subpath))
+        for subpath in ['T', 'T/C', 'T/C/no-extension']:
+            tester(subpath, 'without extension are ambiguous')
+        tester('T/C/something/', 'trailing slash')
diff --git a/stor/utils.py b/stor/utils.py
index a475e119..5d71c215 100644
--- a/stor/utils.py
+++ b/stor/utils.py
@@ -7,6 +7,7 @@
 import shutil
 from subprocess import check_call
 import tempfile
+import warnings
 
 from stor import exceptions
 
@@ -312,6 +313,50 @@ def is_writeable(path, swift_retry_options=None):
     return answer
 
 
+def validate_file_path(dest):
+    """Raises ValueError if dest is an ambiguous or invalid path.
+
+    Args:
+        dest (Path): path to validate
+    Raises:
+        ValueError: if path is ambiguous and is not a filesystem path (see below)
+    Note:
+        if an ambiguous filesystem path and ``raise_on_ambiguous_filesystem_path`` is true, will raise
+        a ValueError as well, otherwise it warns.
+
+    In OBS-land, we cannot differentiate between files and directories, except by convention.
+    Conventionally, directories are either prefixes or zero-sized objects ending with a trailing
+    slash. With POSIX copies, it's conventional to do something like::
+        cp source/myfile.txt dest
+
+    In which case the system checks whether dest is a directory and, if it is, dumps to
+    ``dest/myfile.txt`` or otherwise dumps to ``dest``.
+
+    We can't do that kind of validation and don't want the outcome of a command to depend upon
+    whether there's a file with the same prefix or not. Thus, we raise an error if you try to use
+    `stor.copy()`. (to manipulate files without extensions, you can always use `stor.copytree()`.
+    """
+    import stor.settings
+
+    # we use a try/except so that it's easier for the reader to tell why the exception was
+    # generated.
+    try:
+        if not dest.name:
+            raise ValueError('file paths may not end with trailing slash')
+        if not dest.ext:
+            raise ValueError('file paths without extension are ambiguous')
+    except ValueError as e:
+        if (is_filesystem_path(dest) and
+                not stor.settings.get()['stor']['raise_on_ambiguous_filesystem_path']):
+            warnings.warn(
+                'In stor 2.0, ambiguous filesystem paths will raise an error (%s)' % str(e),
+                # this works for stor.copy, will not look right when the copy method is used
+                stacklevel=4
+            )
+            return
+        raise
+
+
 def copy(source, dest, swift_retry_options=None):
     """Copies a source file to a destination file.
 
@@ -347,18 +392,18 @@ def copy(source, dest, swift_retry_options=None):
             >>> local_p.copy('swift://tenant/container/dir')
             Traceback (most recent call last):
             ...
-            ValueError: OBS destination must be file with extension or directory with slash
+            ValueError: OBS file destinations without extension are ambiguous
     """
     from stor import Path
     from stor.obs import OBSUploadObject
 
     source = Path(source)
+    validate_file_path(source)
     dest = Path(dest)
+    validate_file_path(dest)
     swift_retry_options = swift_retry_options or {}
     if is_obs_path(source) and is_obs_path(dest):
         raise ValueError('cannot copy one OBS path to another OBS path')
-    if is_obs_path(dest) and dest.is_ambiguous():
-        raise ValueError('OBS destination must be file with extension or directory with slash')
 
     if is_filesystem_path(dest):
         dest.parent.makedirs_p()