Revealing mistake: After the headless AT-AT begins to fall over, watch its back leg on the right side of the screen. You can see a broomstick being pushed up from under the set. It raises that foot up in order to ensure that the AT-AT falls to the left side of the screen. (00:33:00)
Revealing mistake: After Chewbacca pulls R2-D2 away from the power socket, he runs away, and you can see white rubber soles under his feet. (01:43:15)
Revealing mistake: In Cloud City, Lando, Leia and Chewbacca are running to the Millennium Falcon, and when then they run into a door after Leia shoots a stormtrooper, a white rubber sole can be seen under one of Chewie's feet. (01:42:40)
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: 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.
Revealing mistake: If you closely when Vader cuts through the three poles at the end of the lightsaber duel, you can see that the saber never actually goes through the poles. (01:45:50)