Corrected entry: You'll have to use frame by frame on this one, and jog it back and forth to see it. When Luke and Han are first arguing about whether or not the Force exists, Luke turns off his light saber. In the frame just before he turns off his light saber, his hands are in very different positions from the frame after he turns it off. Also, an item on the table to his right disappears, and the little sphere that's been firing at him rotates ever so slightly.
rstill
5th Nov 2004
Correction: The requirement to use freeze frame and 'jogging back and forth' invalidates this mistake.