Revealing mistake: Every time the villains' deadly gas, "hydro," is mentioned, the word is badly and very obviously dubbed in, and doesn't match what the actors' lips are saying. This occurred because the original name used, "freon," turned out to be an existing (and patented) refrigerant gas, so the name had to be changed to avoid a trademark lawsuit.
Revealing mistake: When Napoleon Solo arrives at the train station, both the name of the newspaper the man in black is holding, and the name on the station sign are backwards. Additionally, as he arrives by train the man in black is on the left side of the shot. After exiting the train, the man in black appears from the right side of the shot.