Tests sometimes fail on EC2 due to _LockWarner garbage

I've just had two unrelated branches fail on EC2 test with errors about _LockWarner garbage.

In one, all of the devscripts tests failed like:
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ERROR: devscripts.tests.test_sourcecode.TestPlanUpdate.test_trivial (subunit.RemotedTestCase)
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_StringException: Text attachment: garbage
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[<bzrlib.lockable_files._LockWarner object at 0xdbc49d0>]
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In the other, a test which checked for output to stderr failed with:
======================================================================
FAILURE: lp.scripts.tests.test_sphinxdocs.TestSphinxDocumentation.test_docs_build_without_error (subunit.RemotedTestCase)
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_StringException: Text attachment: traceback
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Traceback (most recent call last):
_StringException: Text attachment: traceback
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Traceback (most recent call last):
 File "/var/launchpad/tmp/eggs/testtools-0.9.8-py2.6.egg/testtools/runtest.py", line 169, in _run_user
   return fn(*args, **kwargs)
 File "/var/launchpad/tmp/eggs/testtools-0.9.8-py2.6.egg/testtools/testcase.py", line 499, in _run_test_method
   return self._get_test_method()()
 File "/var/launchpad/test/lib/lp/scripts/tests/test_sphinxdocs.py", line 34, in test_docs_build_without_error
   self.assertEqual('Making output directory...\n', stderr)
 File "/var/launchpad/tmp/eggs/testtools-0.9.8-py2.6.egg/testtools/testcase.py", line 268, in assertEqual
   self.assertThat(observed, matcher)
 File "/var/launchpad/tmp/eggs/testtools-0.9.8-py2.6.egg/testtools/testcase.py", line 345, in assertThat
   % (matchee, matcher, mismatch.describe()))
AssertionError: Match failed. Matchee: "Making output directory...
Exception IndexError: IndexError('list index out of range',) in <bound method _LockWarner.__del__ of <bzrlib.lockable_files._LockWarner object at 0xef9ea50>> ignored
"
Matcher: Equals('Making output directory...\n')
Difference: !=:
reference = 'Making output directory...\n'
actual = "Making output directory...\nException IndexError: IndexError('list index out of range',) in <bound method _LockWarner.__del__ of <bzrlib.lockable_files._LockWarner object at 0xef9ea50>> ignored\n"

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Neither of the branches dealt messed with Bazaar in any way. Julian also reports getting the devscript failures from another branch.