Spurious test failure: TestDNSForwardZoneConfig_GetGenerateDirectives.test_returns_single_entry_for_tiny_network

Just saw this spurious-looking test failure in a totally unrelated branch:

«
FAIL: provisioningserver.dns.tests.test_zoneconfig.TestDNSForwardZoneConfig_GetGenerateDirectives.test_returns_single_entry_for_tiny_network
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/home/jtv/launchpad/maas/curtin-copy/src/provisioningserver/dns/tests/test_zoneconfig.py", line 650, in test_returns_single_entry_for_tiny_network
    self.assertItemsEqual(expected_directives, directives)
  File "/usr/lib/python2.7/unittest/case.py", line 903, in assertItemsEqual
    self.fail(msg)
  File "/usr/lib/python2.7/unittest/case.py", line 412, in fail
    raise self.failureException(msg)
AssertionError: Element counts were not equal:
First has 1, Second has 0: (u'1-1', u'31-254-73-$', u'31.254.73.$')
First has 0, Second has 1: (u'0-1', u'31-254-73-$', u'31.254.73.$')
»

In trunk I get this roughly once every 200 or 300 times I run the tests.