Revealing mistake: Widescreen version: After Luke cuts off Vader's hand, the Emperor says, "Now, fulfill your destiny, and take your father's place at my side." In the next shot, Luke's lightsaber blade goes through the railing, without damaging it. (01:49:15)
Continuity mistake: In the beginning of the scene where Luke is being electrocuted by the Emperor, the Emperor is standing at the bottom of the stairs. When he says, "Young fool... Only now, at the end, do you understand", he is standing in front of the great window. However, in the next shot of the Emperor, he is standing at the bottom of the stairs again, before he walks down to his previous position. (01:50:50)
Continuity mistake: In the bunker, just after R2 gets shot, Han tries to hotwire the door. A second blast door closes, and you can see that it is very flat and cardboard looking. A few moments later, Leia gets shot, and as Han is tending to her, notice that the door is now massively armored and three-dimensional, completely different from the door seen earlier.
Continuity mistake: When Green Leader's starship crashes into the Star Destroyer, Admiral Piett begins to run away. As the shot ends, he is about one meter away from the window. In the next shot, he is several meters away from it. He couldn't have moved that quickly between shots. (01:54:20)
Deliberate mistake: When Luke is to be executed, Jabba says, "Put him in", and there's a shot of the prisoner's skiff. The shot has been flipped, seen from the positions of Lando and the guards, as well as the railing of the skiff.
Continuity mistake: In the beginning of Luke and Vader's lightsaber duel, and also in a few shots right after Luke knocks Vader down the stairs, Vader's cape alternates between being draped behind and over his left shoulder. (01:48:55)
Continuity mistake: In one scene of the battle of Endor, Chewbacca is accompanied by 2 Ewoks while pursuing one of the ATSTs. However soon after their first appearance, one of the Ewoks' costume looks completely different from the original one.
Revealing mistake: In the wide-shot where Luke leaps from the prisoner skiff over to another skiff, you can see the stick used during the filming, which is much shorter than the regular one. The blade is not even glowing. (It is not just simply the hilt of the sabre, it's too long for that.). (00:32:15)
Continuity mistake: When Luke is to be executed, he walks to the edge of the plank, and his feet are side by side as he stops walking. But in the beginning of the next shot, one of his feet is behind the other and we see him take another step. (00:30:55)
Continuity mistake: After Han is released from the carbonite, he falls to the ground, soiling his shirt. In the following shots, the amount of dirt changes continuously between shots. Later, when Han and Luke are brought before Jabba, his shirt is clean.
Continuity mistake: When the little Ewok finds Leia, and she rises up, he points his spear at her and you can see that the front of his leather hood is over the spear's shaft. The shot changes to Leia, the back to the Ewok and now spear is over the hood, the shots change again and now the hood's front is crumbled up between the spear and the Ewok´s body.
Revealing mistake: During the lightsaber duel, Vader throws his at Luke. The catwalk behind Luke explodes, but if you watch in slow-motion there are two explosions: one some inches over where the lightsaber actually hits and one far on the left.