in lib/test/test_os.py, there is a test class, Win32ErrorTests, that 
tests that certain functions return a WindowsError on failure.  And 
indeed they do that, but that is in contradiction with the 
documentation.  From the 2.6 docs:
 Note
 All functions in this module raise OSError in the case of invalid or
 inaccessible file names and paths, or other arguments that have the
 correct type, but are not accepted by the operating system.