We've had several issues where mypy produces incorrect "Cannot instantiate abstract class" errors because of faulty typeshed stubs (e.g., #1379, #1160, #351, #1475). To find more bugs like those, I hacked mypy (JelleZijlstra/mypy@1fa9c65) to error on any class with abstract methods. Here are the results for 3.6, excluding private classes and those clearly intended to be abstract:

• stdlib/3/collections/__init__.pyi:62: error: Class collections.UserDict has abstract attributes ['__delitem__', '__getitem__', '__iter__', '__len__', '__setitem__']
• stdlib/3/collections/__init__.pyi:63: error: Class collections.UserList has abstract attributes ['__delitem__', '__getitem__', '__len__', '__setitem__', 'insert']
• stdlib/3/collections/__init__.pyi:64: error: Class collections.UserString has abstract attributes ['__getitem__', '__len__']
• third_party/2and3/requests/structures.pyi:5: error: Class requests.structures.CaseInsensitiveDict has abstract attributes ['__delitem__', '__getitem__', '__iter__', '__len__', '__setitem__']
• third_party/2and3/pytz/lazy.pyi:6: error: Class pytz.lazy.LazyDict has abstract attributes ['__getitem__', '__iter__', '__len__']
• stdlib/2and3/codecs.pyi:205: error: Class codecs.StreamReaderWriter has abstract attributes ['close', 'fileno', 'flush', 'isatty', 'readable', 'seekable', 'tell', 'truncate', 'writable']
• stdlib/2and3/codecs.pyi:221: error: Class codecs.StreamRecoder has abstract attributes ['__enter__', '__exit__', '__iter__', '__next__', 'close', 'fileno', 'flush', 'isatty', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines']
• stdlib/2and3/bz2.pyi:14: error: Class bz2.BZ2File has abstract attributes ['__enter__', '__exit__', '__iter__', '__next__', 'close', 'fileno', 'flush', 'isatty', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines']
• stdlib/3/urllib/response.pyi:6: error: Class urllib.response.addinfourl has abstract attributes ['__enter__', '__exit__', '__iter__', '__next__', 'close', 'fileno', 'flush', 'isatty', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines']
• stdlib/3/http/cookies.pyi:21: error: Class http.cookies.BaseCookie has abstract attributes ['__delitem__', '__getitem__', '__iter__', '__len__']
• stdlib/3/http/cookies.pyi:31: error: Class http.cookies.SimpleCookie has abstract attributes ['__delitem__', '__getitem__', '__iter__', '__len__']
• stdlib/3.4/tracemalloc.pyi:64: error: Class tracemalloc.Traceback has abstract attributes ['__getitem__', '__len__']
• stdlib/3.4/enum.pyi:7: error: Class enum.EnumMeta has abstract attributes ['__len__']
• stdlib/2and3/distutils/cmd.pyi:7: error: Class distutils.cmd.Command has abstract attributes ['finalize_options', 'initialize_options', 'run']
• stdlib/3/urllib/error.pyi:8: error: Class urllib.error.HTTPError has abstract attributes ['__enter__', '__exit__', '__iter__', '__next__', 'close', 'fileno', 'flush', 'isatty', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines']
• stdlib/3/io.pyi:260: error: Class io.IncrementalNewlineDecoder has abstract attributes ['decode']
• stdlib/3/encodings/utf_8.pyi:3: error: Class encodings.utf_8.IncrementalEncoder has abstract attributes ['encode']
• stdlib/3/encodings/utf_8.pyi:5: error: Class encodings.utf_8.IncrementalDecoder has abstract attributes ['_buffer_decode']
• stdlib/2and3/distutils/command/build_py.pyi:7: error: Class distutils.command.build_py.build_py has abstract attributes ['finalize_options', 'initialize_options', 'run']
• stdlib/2and3/distutils/command/build_py.pyi:8: error: Class distutils.command.build_py.build_py_2to3 has abstract attributes ['finalize_options', 'initialize_options', 'run']
• stdlib/2and3/distutils/command/bdist_msi.pyi:5: error: Class distutils.command.bdist_msi.bdist_msi has abstract attributes ['finalize_options', 'initialize_options', 'run']
• stdlib/3.4/asyncio/events.pyi:207: error: Class asyncio.events.BaseDefaultEventLoopPolicy has abstract attributes ['get_child_watcher', 'set_child_watcher']
• stdlib/2and3/asynchat.pyi:12: error: Class asynchat.async_chat has abstract attributes ['collect_incoming_data', 'found_terminator']
• third_party/2and3/requests/packages/urllib3/response.pyi:25: error: Class requests.packages.urllib3.response.HTTPResponse has abstract attributes ['__enter__', '__exit__', '__iter__', '__next__', 'isatty', 'readline', 'readlines', 'seek', 'seekable', 'truncate', 'writable', 'write', 'writelines']
• third_party/2and3/requests/cookies.pyi:44: error: Class requests.cookies.RequestsCookieJar has abstract attributes ['__len__']

I believe most of these aren't intended to be abstract.