Ivan Igor says that Jean Paul Marat's assassin, Charlotte Corday, was his mistress. Not so - they had never met until she came to his office posing as a courier and quickly stabbed him to death. After her execution a few days later, she was found to be virgo intacta.
When Florence is studying the wax figure of Joan of Arc she glances down and sees a tag laying in a crate, when she picks it up she's not wearing gloves but is wearing them when she puts the tag in her pocket.
During the opening sequence when Igor is showing off his "children," he points out a tableau entitled 'Mother Love' which features a woman nursing an infant surrounded by several children. In long shots the figure of the mother wears a head scarf pulled back exposing her shoulder, while in the close-up the scarf is pulled forward over her bosom (presumably to deemphasize the fact that she is breast feeding).
Florence is facing the figure of Joan Of Arc when she puts the tag in her pocket, appears to be sitting sideways when we see the hand pull the tag out of her pocket, then appears to be facing the figure again in the shot after.
In the opening scene of the film, our first shot of Igor is a long shot showing him sculpting a figure with his right hand while he holds a cigarette in his left hand; in the next shot, both of his hands are molding the figure and his cigarette is dangling from his lips.
Early in the movie, the shadow of the camera is visible on the road as the man crosses the street, and then again when Charlotte walks into the police station, this time on the backs of chairs and on the table.
The wax statues are played by real people (see trivia). Marie Antoinette, Joan of Arc, and Queen Victoria breathe, twitch, flinch, and blink at several points throughout the movie. One statue, knocked over in the London brawl, withdraws her legs from the fighters' path.
The goof item below may give away important plot points.
The worker who unloads the Voltaire statue in New York is played by the same actor who poses as Judge Ramsey in the press photo, even though the Voltaire statue is supposed to be Judge Ramsey's corpse.