Spurious test failure: test_api_nodegroup.py:TestUpdateMacClusterInterfaces.test_updates_mac_cluster_interfaces

Spurious failure in src/maasserver/tests/test_api_nodegroup.py:TestUpdateMacClusterInterfaces.test_updates_mac_cluster_interfaces

======================================================================
FAIL: maasserver.tests.test_api_nodegroup.TestUpdateMacClusterInterfaces.test_updates_mac_cluster_interfaces
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/home/ubuntu/maas/src/maasserver/tests/test_api_nodegroup.py", line 855, in test_updates_mac_cluster_interfaces
    self.assertEqual(mac_addresses, results)
  File "/home/ubuntu/maas/eggs/testtools-0.9.34-py2.7.egg/testtools/testcase.py", line 324, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/ubuntu/maas/eggs/testtools-0.9.34-py2.7.egg/testtools/testcase.py", line 414, in assertThat
    raise MismatchError(matchee, matcher, mismatch, verbose)
MismatchError: !=:
reference = {<MACAddress: 64:6b:74:9c:f8:bb>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-yTiW7a>,
 <MACAddress: 8c:5a:55:a8:f9:2f>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-yTiW7a>,
 <MACAddress: e1:8f:3a:f8:2b:a8>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-yTiW7a>,
 <MACAddress: ec:a3:8d:07:c5:9c>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-JxrEF7>}
actual = {<MACAddress: e1:8f:3a:f8:2b:a8>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-yTiW7a>,
 <MACAddress: 64:6b:74:9c:f8:bb>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-yTiW7a>,
 <MACAddress: ec:a3:8d:07:c5:9c>: <NodeGroupInterface 9c19b2fc-0280-11e4-96d2-fa163edde9d5,interface-JxrEF7>}

-------------------- >> begin captured logging << --------------------
maasserver.eventloop: INFO: Starting event loop in process 31136
twisted: INFO: Log opened.
twisted: INFO: /home/ubuntu/maas/src/metadataserver/fields.py:58: exceptions.DeprecationWarning: object.__init__() takes no parameters
--------------------- >> end captured logging << ---------------------

How to reproduce:
run-one-until-failure ./bin/test.maas src/maasserver/tests/test_api_nodegroup.py:TestUpdateMacClusterInterfaces.test_updates_mac_cluster_interfaces