Create a symlink pointing to itself:

ln -s infinite infinite

Run os.path.realpath() on it, and it recurses
infinitely (until the stack limit is hit):

>>> import os
>>> os.path.realpath('/home/amk/infinite')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.3/posixpath.py", line 416, in
realpath
    return realpath(newpath)

os.path.realpath() should be fixed; /home/amk/infinite
is a perfectly good path, though it can't be followed.