Random failures

The C++ test test_from_cxx_disable is randomly failing against the new environment we're deploying, but just on OSX. The error is the following:

Traceback (most recent call last):
  File "/Users/buildbot/work/gitlab-ci/f6f95b82/2/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/buildbot/work/gitlab-ci/f6f95b82/2/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/bob/core/test_logging.py", line 80, in test_from_cxx_disable
    assert _test_output_disable(), "The C++ test function returned false, indicating an (unknonw) error"
AssertionError: The C++ test function returned false, indicating an (unknonw) error

It looks like there is an uninitialised variable somewhere. But where?