Battle of the Giants! - S4-E14
Other mistake: Mainwaring hangs his new cap on a hook on the inside of his office door, the rest of the platoon are having bayonet practice in the hall. Pike is running with the bayonet but misses the dummy and his bayonet goes through the office door and Mainwaring's cap. The bayonet only goes through the cap once, but there are two tears in the cap.