Hornblower: The Frogs and the Lobsters (1999)

Plot hole: When it becomes apparent that the British will have to retreat, Matthews is instructed to disable the cannon. He reports to the Mr. Kennedy that he has spiked the cannon (a method of temporarily rendering the gun useless) and that he had, "blown the trunnions off for good measure." This would have turned the cannon into rather large boat anchors, so why bother investing valuable time in spiking them?