Revealing mistake: After King Kong destroys the train track of the oncoming train, in the POV shot directly behind the train operator while he watches Kong rise from the broken track, the small model's support control bar is visible at the right as it moves up and down. The next shot is a close-up of the shocked motorman, and after he attempts to stop the train, there's another shot facing Kong with the control bar visible for a moment. (01:31:50)
Revealing mistake: When the woman at the bar brings Jack the mustard which she forgot, Jack puts his napkin in front of his mouth. A fraction of a second before that, you can see the down-side of his face which should be invisible. Instead of nothingness you see it's all black. That is material used for the special effects, but they didn't 'make it invisible'. (00:06:50)
Revealing mistake: In an overhead view of Oliver Hardy going down the sawdust chute, you can see it is a stunt double.
Revealing mistake: When Denham is rowing the boat to the island - an obvious rear projection - the oars never touch the water.
Revealing mistake: When Pinky runs into the mirror, causing it to shatter, you can that the mirror does not shatter until Pinky falls onto the floor.
Revealing mistake: At the beginning of the movie when he is talking to the figure of Marie Antoinette if you look really carefully you can see the actress who's portraying her ever so slightly move.