Revealing mistake: Towards the end when the Phantom's lair is collapsing, you can see bricks falling into the sewer water and floating. Obviously they were made out of light-weight material and not out of actual stone.
Revealing mistake: In the scene where a British soldier (Williams) and Sgt. Joe Gunn (Humphrey Bogart) are in a trench, the British soldier is shot dead by a German sniper. Joe Gunn moves in for a closer look and it can be seen that the "dead" soldier is breathing, his shoulder is moving. (01:15:30)