Deliberate mistake: In the scene where the FBI agents are transporting Lester Cowans in a car and telling him all his friends have confessed, for about three seconds the scenery goes by backwards. For some reason, the shot is played in reverse.
Deliberate mistake: In the scene where Mike is in the car and Chucky attacks him, pulling the skipping ropes around his neck and stabbing a knife through the carseat etc., in all the wideshots of the car driving, the stunt driver wearing what looks like a black woolly hat replacing Mike is blatant. (00:54:15 - 00:55:45)
Deliberate mistake: For Hans Gruber's iconic death scene, Alan Rickman's fall was filmed at high speed (for slow-motion playback) against a green screen, and the skyscraper perspective footage was added later as background. However, while Rickman falls away from the camera in slow motion, papers are fluttering around him in the background at normal speed. This was done deliberately to make the shot even more surreal.