This has been happening recently on our Windows Server 2008 buildbot:


======================================================================
FAIL: test_first_completed (test.test_concurrent_futures.ThreadPoolWaitTests)
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Traceback (most recent call last):
  File "c:\buildslave-py3k\3.x.curtin-win2008-amd64\build\lib\test\test_concurrent_futures.py", line 252, in test_first_completed
    self.assertEqual(set([future1]), done)
AssertionError: Items in the second set but not the first:
<Future at 0x19899f60 state=finished raised AssertionError>

======================================================================
FAIL: test_first_exception (test.test_concurrent_futures.ThreadPoolWaitTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\buildslave-py3k\3.x.curtin-win2008-amd64\build\lib\test\test_concurrent_futures.py", line 293, in test_first_exception
    self.assertEqual(set([future1, future2]), finished)
AssertionError: Items in the second set but not the first:
<Future at 0x164e5878 state=finished raised AssertionError>

======================================================================
FAIL: test_first_exception_some_already_complete (test.test_concurrent_futures.ThreadPoolWaitTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\buildslave-py3k\3.x.curtin-win2008-amd64\build\lib\test\test_concurrent_futures.py", line 323, in test_first_exception_some_already_complete
    future1]), finished)
AssertionError: Items in the second set but not the first:
<Future at 0x16f9b948 state=finished raised AssertionError>

======================================================================
FAIL: test_timeout (test.test_concurrent_futures.ThreadPoolWaitTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\buildslave-py3k\3.x.curtin-win2008-amd64\build\lib\test\test_concurrent_futures.py", line 442, in test_timeout
    future1]), finished)
AssertionError: Items in the second set but not the first:
<Future at 0xd0efe28 state=finished raised AssertionError>


(see e.g. http://www.python.org/dev/buildbot/all/builders/AMD64%20Windows%20Server%202008%203.x/builds/306/steps/test/logs/stdio )