 Glenn Strange (qv) was playing the Frankenstein monster, but  during shooting one day he tripped over a camera cable and broke his  ankle. Lon Chaney Jr. (qv) (playing the Wolf Man) wasnt working  that day, so he put on the Frankenstein makeup/outfit and filled in  for Strange in one scene where Dr. Mornay gets thrown through the  window. So Chaney wound up playing two monsters in this movie.