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.
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.