Continuity: 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: In the scene where 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.