$ python3.7 -S
>>> import posixpath
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "--/Lib/posixpath.py", line 13, in <module>
    import os
  File "--/Lib/os.py", line 92, in <module>
    from os.path import (curdir, pardir, sep, pathsep, defpath, extsep, altsep,
ImportError: cannot import name 'curdir' from 'posixpath' (--/Lib/posixpath.py)

Whether this counts as a bug or not is debatable. It could be argued that you shouldn't be importing 'posixpath' directly, in which case it ought to be called '_posixpath', but I guess it is too late to be changing the name.