Other mistake: During the final confrontation with Vader on the Death Star, Luke bashes a catwalk railing with his light saber several times, but there are no burns, marks, or indentations on it at all. Considering the power of a light saber (Qui Gon melted a steel door in Phantom Menace), some damage should be visible. Only when he hacks off Vader's hand does the light saber slice through the rail.
Continuity mistake: In the first shot of the hologram of Endor and the Death Star, the Death Star orbits Endor in an counter-clockwise direction. In the subsequent shots, the Death Star orbits in a clockwise direction.
Continuity mistake: When Boba Fett falls into the mouth of the Sarlaac, he disappears just before the beak touches him. (00:33:45)
Continuity mistake: After Vader's shuttle lands on Endor, as the AT-AT is walking towards one of the ramps, you can see that the ramps are connected right next to the lift, under the landing pad. But when Vader exits the lift, there's a circular platform going around the lift, which wasn't there in the previous shots. (01:19:40)
Continuity mistake: When Vader throws his lighsabre, it hits one of the supports holding the catwalk, but in the next shot, it is nowhere to be seen. (01:45:50)
Continuity mistake: When Bib Fortuna attempts to persuade R2-D2 to show him the message to Jabba, his hands keep changing position between shots. (00:07:20)
Continuity mistake: When Lando says goodbye to Han, after Ackbar's speech, Lando's lapel pin/medal swiches shoulders several times. (00:51:40)
Continuity mistake: Luke tells Threepio to tell the Ewoks that if our heroes are not set free, that he will use his magic. If you look on the right of the screen, Artoo is standing untied, even though at that point in the movie he had not been untied yet.
Continuity mistake: Just after R2-D2 cuts the net that holds him, Luke, Han, Chewbacca, & C-3PO, you can see that before the Ewoks pop up out of nowhere to see their captured victims, one Ewok just stands there in plain sight waiting for the others to reveal themselves.