Mystery of the Wax Museum (1933)

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.

Ivan Igor: If my curiosity is not too great, would you mind telling me what manner of animal it is you are designing?
Ralph Burton: This is one of the Athenian girls for the bacchanal.
Ivan Igor: It would be interesting to know, young man, where and when you studied anatomy.