Whilst testing the updated management console, attempt was made to clear the queues used previously as destinations to test copying and moving messages between queues. The originating queue had previously been cleared without issue, however attempting to clear the queues to which the messages were moved/copied resulted in a FailedDequeueException.
This may relate to a difference in treatment of persistent messages between their initial delivery to the broker, and when they are moved/copied. Currently all persistent messages are entered into the store on arrival at the broker, regardless of whether the queue they are destined for is durable. When moving/copying, the queue durability is also checke, and so the messages that threw the FailedDequeueException (but apprently were dequeued) had not been entered into the store for the destination queue as the queue was not durable.