I'm got this once in devstack, using a version of nova from early June - it could be related as it failed to destroy an instance:

2014-07-08 23:03:09.743 16994 ERROR nova.virt.libvirt.driver [-] [instance: 556caeed-b676-440a-b56b-a16e51ad3768] During wait destroy, instance disappeared.
2014-07-08 23:03:10.133 ERROR oslo.messaging.rpc.dispatcher [req-469b8c35-3c3b-43e4-beac-aa51acff7598 None None] Exception during message handling: Circular reference detected
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher Traceback (most recent call last):
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 133, in _dispatch_and_reply
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     incoming.message))
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 176, in _dispatch
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return self._do_dispatch(endpoint, method, ctxt, args)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 122, in _do_dispatch
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     result = getattr(endpoint, method)(ctxt, **new_args)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/exception.py", line 88, in wrapped
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     payload)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/openstack/common/excutils.py", line 82, in __exit__
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     six.reraise(self.type_, self.value, self.tb)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/exception.py", line 71, in wrapped
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return f(self, context, *args, **kw)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/compute/manager.py", line 285, in decorated_function
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     pass
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/openstack/common/excutils.py", line 82, in __exit__
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     six.reraise(self.type_, self.value, self.tb)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/compute/manager.py", line 271, in decorated_function
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return function(self, context, *args, **kwargs)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/compute/manager.py", line 335, in decorated_function
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     function(self, context, *args, **kwargs)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/compute/utils.py", line 436, in __exit__
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     exc_tb=exc_tb)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/objects/base.py", line 113, in wrapper
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     args, kwargs)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/opt/stack/new/nova/nova/conductor/rpcapi.py", line 355, in object_class_action
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     objver=objver, args=args, kwargs=kwargs)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/client.py", line 150, in call
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     wait_for_reply=True, timeout=timeout)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/transport.py", line 90, in _send
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     timeout=timeout)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/amqpdriver.py", line 412, in send
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return self._send(target, ctxt, message, wait_for_reply, timeout)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/amqpdriver.py", line 385, in _send
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     msg = rpc_common.serialize_msg(msg)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/common.py", line 317, in serialize_msg
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     _MESSAGE_KEY: jsonutils.dumps(raw_msg)}
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/openstack/common/jsonutils.py", line 164, in dumps
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return json.dumps(value, default=default, **kwargs)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/lib/python2.7/json/__init__.py", line 238, in dumps
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     **kw).encode(obj)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/lib/python2.7/json/encoder.py", line 201, in encode
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     chunks = self.iterencode(o, _one_shot=True)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher   File "/usr/lib/python2.7/json/encoder.py", line 264, in iterencode
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher     return _iterencode(o, 0)
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher ValueError: Circular reference detected
2014-07-08 23:03:10.133 16994 TRACE oslo.messaging.rpc.dispatcher 
2014-07-08 23:03:10.140 ERROR oslo.messaging._drivers.common [req-469b8c35-3c3b-43e4-beac-aa51acff7598 None None] Returning exception Circular reference detected to caller
2014-07-08 23:03:10.141 ERROR oslo.messaging._drivers.common [req-469b8c35-3c3b-43e4-beac-aa51acff7598 None None] ['Traceback (most recent call last):\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 133, in _dispatch_and_reply\n    incoming.message))\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 176, in _dispatch\n    return self._do_dispatch(endpoint, method, ctxt, args)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 122, in _do_dispatch\n    result = getattr(endpoint, method)(ctxt, **new_args)\n', '  File "/opt/stack/new/nova/nova/exception.py", line 88, in wrapped\n    payload)\n', '  File "/opt/stack/new/nova/nova/openstack/common/excutils.py", line 82, in __exit__\n    six.reraise(self.type_, self.value, self.tb)\n', '  File "/opt/stack/new/nova/nova/exception.py", line 71, in wrapped\n    return f(self, context, *args, **kw)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 285, in decorated_function\n    pass\n', '  File "/opt/stack/new/nova/nova/openstack/common/excutils.py", line 82, in __exit__\n    six.reraise(self.type_, self.value, self.tb)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 271, in decorated_function\n    return function(self, context, *args, **kwargs)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 335, in decorated_function\n    function(self, context, *args, **kwargs)\n', '  File "/opt/stack/new/nova/nova/compute/utils.py", line 436, in __exit__\n    exc_tb=exc_tb)\n', '  File "/opt/stack/new/nova/nova/objects/base.py", line 113, in wrapper\n    args, kwargs)\n', '  File "/opt/stack/new/nova/nova/conductor/rpcapi.py", line 355, in object_class_action\n    objver=objver, args=args, kwargs=kwargs)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/client.py", line 150, in call\n    wait_for_reply=True, timeout=timeout)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/transport.py", line 90, in _send\n    timeout=timeout)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/amqpdriver.py", line 412, in send\n    return self._send(target, ctxt, message, wait_for_reply, timeout)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/amqpdriver.py", line 385, in _send\n    msg = rpc_common.serialize_msg(msg)\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/_drivers/common.py", line 317, in serialize_msg\n    _MESSAGE_KEY: jsonutils.dumps(raw_msg)}\n', '  File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/openstack/common/jsonutils.py", line 164, in dumps\n    return json.dumps(value, default=default, **kwargs)\n', '  File "/usr/lib/python2.7/json/__init__.py", line 238, in dumps\n    **kw).encode(obj)\n', '  File "/usr/lib/python2.7/json/encoder.py", line 201, in encode\n    chunks = self.iterencode(o, _one_shot=True)\n', '  File "/usr/lib/python2.7/json/encoder.py", line 264, in iterencode\n    return _iterencode(o, 0)\n', 'ValueError: Circular reference detected\n']