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I have observed that when running unit tests using unittest's test discovery, unittest can simultaneously (1) modify sys.path unnecessarily (by adding a path that is already in sys.path with a different case), and (2) modify sys.path by adding a path of the "wrong" case.
This occurs for me on Mac OS X with the default case-insensitive file system.
If the path--
'/Users/chris/Dev/python/my_package'
is already in sys.path, running unittest's test discovery will prepend sys.path with the following:
'/Users/chris/dev/python/my_package'
Aside from causing unnecessary modifications of sys.path, this also causes an issue in coverage, for example:
https://bitbucket.org/ned/coveragepy/issue/348
The relevant code is here in unittest/loader.py (with `top_level_dir` starting out as os.curdir):
 top_level_dir = os.path.abspath(top_level_dir)
 if not top_level_dir in sys.path:
 # all test modules must be importable from the top level directory
 # should we *unconditionally* put the start directory in first
 # in sys.path to minimise likelihood of conflicts between installed
 # modules and development versions?
 sys.path.insert(0, top_level_dir)
 self._top_level_dir = top_level_dir
(from https://hg.python.org/cpython/file/75ede5bec8db/Lib/unittest/loader.py#l259 )
The issue occurs when os.path.abspath(top_level_dir) is already in sys.path but with a different case. (Note that if os.path.abspath() returned a path with the "right" case, then the unittest code would be okay.)
See also these two threads regarding obtaining the correct case for a path:
1. https://mail.python.org/pipermail/python-dev/2010-September/103823.html
2. https://mail.python.org/pipermail/python-ideas/2010-September/008153.html 

As an aside, should that be sys.path.insert(1, X)? As 0 has a special meaning (I've often thought this is a slightly odd convention).
Another aside: I noticed this because I was looking to write a module finder, and thought I could extract one out of this function. It would be nice to have a module-crawler function, and write discover() in terms of that.