Seen on the AMD64 Mountain Lion Optimized [SB] 2.7 buildbot
(http://buildbot.python.org/all/builders/AMD64%20Mountain%20Lion%20Optimized%20%5BSB%5D%202.7):

test test_doctest crashed -- <type 'exceptions.AssertionError'>: filter ('backquote not supported', SyntaxWarning) did not catch any warning
Traceback (most recent call last):
  File "./Lib/test/regrtest.py", line 896, in runtest_inner
  File "/Users/cpython/buildslave/optimized/2.7.snakebite-mountainlion-amd64-optimized/build/Lib/test/test_doctest.py", line 2684, in test_main
    test_support.run_doctest(test_doctest, verbosity=True)
  File "/Users/cpython/buildslave/optimized/2.7.snakebite-mountainlion-amd64-optimized/build/Lib/contextlib.py", line 24, in __exit__
    self.gen.next()
  File "/Users/cpython/buildslave/optimized/2.7.snakebite-mountainlion-amd64-optimized/build/Lib/test/test_support.py", line 641, in _filterwarnings
    missing[0])
AssertionError: filter ('backquote not supported', SyntaxWarning) did not catch any warning

The same failure is seen in test_syntax and test_zipimport_support.

That buildbot builds the interpreter --with-pydebug but runs the test suite with -OO:

./python.exe -Wd -3 -E -tt -OO -R ./Lib/test/regrtest.py -uall -rwW

I am able to reproduce the buildbot failures when run as above.  Without "-OO", the tests no longer fail.