Continuity mistake: If you look carefully at the monster (Frankenstein) you will see that his face is different in the ending scenes. The reason is, during the movie you see the monster take the woman doctor and throw her out the laboratory window. You then see the monster kind of stumble, which he really did and broke his ankle. So in the final scenes they replaced Lon Chaney Jr. as wolfman with a stuntman. and Lon Chaney Jr. finished the movie as the monster.
Factual error: When Dracula (Bela Lugosi) is about to bite the neck of his female assistant you can plainly see him in her vanity mirror and as all good horror fans know...you can't see a vampire in a mirror..especially Dracula.
Visible crew/equipment: During the scene when Lou Costello is reading the narrative on Count Dracula we see the coffin door open and the candle on top of the box starts to decend. By the angle of the coffin door the candle would have easily fallen to the ground. If you look carefully you can see a thin black cord attached to the candle.
Factual error: In no place on Earth does the full moon occur five times in a row, as it is in the movie.
Continuity mistake: In a shot early in the movie, Costello is retrieving a bag for a customer. When he pulls the bag from the middle of the cart, it falls below other bags and packages. But when the shot changes and Abbott asks which bag is the customer's, Costello points to a bag on top of all the others.
Other mistake: Dracula and the Wolfman fall from a balcony, apparently to their deaths. This is not how to kill either a vampire or a werewolf.
Other mistake: When Abbott and Costello are bringing the first crate (Dracula) into the house of horrors, Abbott tells Costello twice to help with moving the crate. The first time Abbott says to help pull the crate, which is correct. The second time, Abbott says to help push, even though Costello is still pulling.