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.
At first, Lou Costello did not want to be in the film because he thought the movie was too silly. But when Universal (the company that made the film) offered him $50,000 to be in the movie, Costello said he would be delighted.