Revealing mistake: When Holmes kills the guineapig-human hybrid, the entire scene is ridiculous. In the first shot, the guineapig has tiny human hands that look to be about a half-inch at most. Holmes then steps on it, and in that shot it appears to be a different type of rodent entirely. Then we see the human hands under his shoe flexing in pain, and they look to be significantly bigger than before based on the scale of his shoe... at least 5X bigger than they were.
Return of the Fly (1959)
1 mistake
Directed by: Edward Bernds
Starring: Vincent Price, Brett Halsey, David Frankham, John Sutton
Trivia: Vincent Price reportedly signed on because he was impressed with the script. However, shortly after signing on, the script was hastily re-written, and the final draft bore little resemblance to the one Price had read when he signed on.