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This happens selectively (and intermittently) on the Solaris buildbot:
test_pass_fds (test.test_subprocess.POSIXProcessTestCase) ... -- maxfd = 256
[36787 refs]
 -- fds that should have been closed: {5, 6, 7, 8, 9, 10, 11, 12, 13}
 -- fds that remained open: {0, 1, 2, 3, 5}
FAIL
test_pass_fds (test.test_subprocess.ProcessTestCasePOSIXPurePython) ... -- maxfd = 256
[36787 refs]
 -- fds that should have been closed: {5, 6, 7, 8, 9, 10, 11, 12, 13}
 -- fds that remained open: {0, 1, 2, 3, 5}
FAIL
As you see (thanks to debug output), all fds have been closed except number 5.
(http://www.python.org/dev/buildbot/all/builders/sparc%20solaris10%20gcc%203.x/builds/2466/steps/test/logs/stdio)