Revealing mistake: In the original (not Special Edition): When the AT-ATs are attacking the Hoth base, they do a camera angle from inside Luke's cockpit. As his snowspeeder is passing through the legs of an AT-AT, part of the cockpit becomes translucent and you can slightly see the AT-AT through a metal bar.
Revealing mistake: After C-3PO gets shot, as Chewbacca walks over to the door, his right foot crushes one of the steps on a small set of stairs. (01:19:25)
Revealing mistake: During the escape from Cloud City, watch as C-3PO's head strikes a lamp on the hull of the Millennium Falcon. It bends out of shape.
Revealing mistake: Right after Luke and Darth fight for the first time, Darth gets kicked off of the Carbonite freezer. Then Luke jumps down to fight him again. After he jumps, you hear what sounds suspiciously like a trampoline. Then in the lower right screen you can see Mark Hamill's head pop back up into frame. Widescreen only, although the sound is audible on the 4:3 version too.