Revealing mistake: When Tom is startled to hear "Rock A Bye Baby" on the phonograph, the whiskers on the left side of his face disappear for a frame or two.
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 this scene we see Cheetah jumping down from a tree to the ground. In the next shot we see Cheetah and a circle of light moves, which indicates they were using a mirror or even a light for extra lighting to brighten up the shot. This happens very briefly and is very noticeable.
Revealing mistake: When the officials arrive at the Talbot family tomb to investigate the recent events, they find the crypt door unlocked. As they start to enter, the door opens on its own. When the camera cuts to an angle from inside the crypt, a thick cord can be seen pulling the door open.
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)
Revealing mistake: When the mice are feeding the cat, the cat sits up and a mouse passes through his head.