Continuity mistake: When Freddy and Janet arrive at Lawrence's mansion as his house guests, they arrive at night. Lawrence takes them upstairs and asks Janet to waltz with him in his ballroom. The scene takes place only a few minutes after their arrival. As they begin to dance, Freddy crawls to the door and starts moaning. Lawrence then shuts the ballroom door in his face and resumes dancing with Janet. As the shot cuts back to them dancing, a large window is plainly in view and daylight is streaming in brightly through the sheer curtains.
Continuity mistake: Right before Steve Martin and Michael Caine make their bet, Steve walks up to Michael sitting at a little white table. Every time the camera goes back to face Michael his handkerchief in the breast pocket of his jacket goes from sticking way out of his jacket, to barely visible.