North By Northwest - Coat Pocket Flap.
At exactly 40:00 into the movie, the character played by Leo G. Carroll (the Professor), stands up from behind a table. The flap of the right front pocket of his suit coat is tucked into the pocket while the flap of the left pocket is out. The scene tracks him around the room, cuts away and when it comes back to him at 40:18 the pocket flap is out. It stays that way for the remainder of the scene. Also, at the end of the scene, his tie has a spot or stain on it that was not there at the beginning when the flap was tucked in.