When using an assert such as self.assertEquals(tester(), expected), an error message suggests wrong argument order (as in, not the order in examples in the documentation). Apparently this is confusing enough that OpenStack even opened a whole bunch of bugs and filed patches to swap the order in the tests:
 https://bugs.launchpad.net/ceilometer/+bug/1277104

If the code is:

import testtools

class TestConfigTrueValue(testtools.TestCase):
    def test_testEquals(self):
        reference = "reference-0123456789012345678901234567890123456789"
        def function_under_test():
            return "actual-0123456789012345678901234567890123456789"
        self.assertEquals(function_under_test(), reference)

then running it yields

[zaitcev@guren xxx]$ python3 -c 'import nose; nose.main()'
F
======================================================================
FAIL: testic.TestConfigTrueValue.test_testEquals
----------------------------------------------------------------------
testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/q/zaitcev/tmp/xxx/testic.py", line 10, in test_testEquals
    self.assertEquals(function_under_test(), reference)
  File "/usr/lib/python3.3/site-packages/testtools/testcase.py", line 322, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/usr/lib/python3.3/site-packages/testtools/testcase.py", line 417, in assertThat
    raise MismatchError(matchee, matcher, mismatch, verbose)
testtools.matchers._impl.MismatchError: !=:
reference = 'actual-0123456789012345678901234567890123456789'
actual    = 'reference-0123456789012345678901234567890123456789'