If a path has a non-existent anchor, Path.mkdir can get into a RecursionError as it tries to recursively create the parent. I expect a more sane error.

This is readily reproducible in Windows with Path('Z:').mkdir(parents=True)

Example execution:

Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 08:06:12) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> Path('Z:').mkdir(parents=True)
Traceback (most recent call last):
  File "C:\Python36\lib\pathlib.py", line 1231, in mkdir
    self._accessor.mkdir(self, mode)
  File "C:\Python36\lib\pathlib.py", line 388, in wrapped
    return strfunc(str(pathobj), *args)
FileNotFoundError: [WinError 3] The system cannot find the path specified: 'Z:'

During handling of the above exception, another exception occurred:

...

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python36\lib\pathlib.py", line 1238, in mkdir
    self.parent.mkdir(parents=True)
  File "C:\Python36\lib\pathlib.py", line 1238, in mkdir
    self.parent.mkdir(parents=True)
  File "C:\Python36\lib\pathlib.py", line 1238, in mkdir
    self.parent.mkdir(parents=True)
  [Previous line repeated 989 more times]
  File "C:\Python36\lib\pathlib.py", line 1231, in mkdir
    self._accessor.mkdir(self, mode)
  File "C:\Python36\lib\pathlib.py", line 388, in wrapped
    return strfunc(str(pathobj), *args)
RecursionError: maximum recursion depth exceeded

Might be worth checking if this is resolved with bpo-29079, which is already in for 3.6.1.

This case is similar to bpo-29079. It should only attempt to make the parent to handle ENOENT if self and self.parent aren't equal. Here's the snippet from Path.mkdir:

try:
    self._accessor.mkdir(self, mode)
except FileExistsError:
    if not exist_ok or not self.is_dir():
        raise
except OSError as e:
    if e.errno != ENOENT:
        raise
    self.parent.mkdir(parents=True)
    self._accessor.mkdir(self, mode)

New changeset 8061d0967988 by Steve Dower in branch '3.5':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
https://hg.python.org/cpython/rev/8061d0967988

New changeset 3de58a54ed98 by Steve Dower in branch '3.6':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
https://hg.python.org/cpython/rev/3de58a54ed98

New changeset 09c018897fb3 by Steve Dower in branch 'default':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
https://hg.python.org/cpython/rev/09c018897fb3

New changeset 661c40ba59855ebe4967424fcabd41be6f799137 by Steve Dower in branch 'master':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
661c40b

New changeset 77da63372461ddeb82dbfdef86e702e43e24e1da by Steve Dower in branch 'master':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
77da633

New changeset c05ffe646dc009c01365db917f0ca6b738fda69b by Steve Dower in branch 'master':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
c05ffe6

New changeset 661c40ba59855ebe4967424fcabd41be6f799137 by Steve Dower in branch '3.6':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
661c40b

New changeset 77da63372461ddeb82dbfdef86e702e43e24e1da by Steve Dower in branch '3.6':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
77da633

New changeset 661c40ba59855ebe4967424fcabd41be6f799137 by Steve Dower in branch '3.5':
Issue bpo-29416: Prevent infinite loop in pathlib.Path.mkdir
661c40b