Continuity mistake: When Don Knotts is in the old Simmons mansion, he falls down a chute of some kind. If you watch closely you will see the stunt person looks nothing like Don Knotts. It is a younger man with very straight hair.
Continuity mistake: In the scene where Paul Newman seduces Shelly Winters in the nightclub, they're dancing to the sounds of a brass band, even though the musicians are all holding guitars.
Continuity mistake: On the airplane, Rock Hudson takes a pillow from the stewardess, and puts it behind his head. They cut away to the stewardess, and back to Hudson, and the pillow is gone. They cut to the stewardess again, then back to Hudson, and the pillow reappears.
Continuity mistake: When the German agents dig up Gromek's buried motorcycle and are pulling it out of the hole in the ground, it is completely clean and in perfect condition with no dirt, scratches, or dents on it.