I got this test failure while building Python 2.2:
test test_email failed -- Traceback (most recent call
last):
  File "./Lib/test/test_email.py", line 935, in
test_formatdate
    self.assertEqual(gdate, matchdate)
  File
"/fs/home/mount/home/prj/b/Python-2.2/Lib/unittest.py",
line 286, in failUnlessEqual
    raise self.failureException, \
AssertionError: 'Fri, 09 Nov 2001 17:33:30 -0000' !=
'Fri, 09 Nov 2001 17:33:52 -0000'

This happens because the test assumes that the system
clock is a count of non-leap seconds since the epoch.
This is a common configuration, but it renders some clock 
values ambiguous, and complicates interval calculations.  
So my clock counts *all* seconds since the epoch.  It
would be nice if the test could handle both cases, by
checking the broken-down values around a leap second, or
by checking that the calculated string matches either of
the two possibilities.