Revealing mistake: When Luke and Vader enter the Emperor's throne room, in the shot from behind, you can see the stairs through Luke. (01:26:30)
Revealing mistake: In the battle on Endor, there is a shot of some Ewoks on a catapult and the at-st fires at them. Notice the second time they get fired at, the blast hits the catapult. Just before the shot ends, notice there is no damage to it. (01:40:10)
Revealing mistake: When Vader throws the lightsaber at Luke and the platform collapses, the point where the platform's columns connect to the ceiling have expanding circles of smoke coming from them as the columns detach from the ceiling, created by the charges used to sever the platform. (01:50:20)
Revealing mistake: During the speeder bike chase scenes, speeders bikes don't affect any vegetation, even though they're flying near the ground at incredible speed.
Revealing mistake: When Luke is being fed to the Rancor, in Jabba the Hutt's dungeon, there are black outlines around the beast's legs, from the composite's blue screen special effect. This was edited out in the special edition rerelease.
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)
Revealing mistake: After the Rebels break into the bunker, a Rebel in the background punches an officer. But if you watch closely or play it in slow motion, the Rebel's hand never comes into contact with the officer's face. (01:33:25)
Revealing mistake: In the scene where Han and Lando are talking in the hangar after the big meeting about the new Death Star, right when the scene opens and the shot pans down, notice how half the hangar is real and the other half is a matte painting. (00:49:30)
Revealing mistake: (Widescreen Only) The rancor puppeteer's arm can be seen on the far right side of the screen as it starts poking at a hiding Luke.
Revealing mistake: In one quick scene when Luke's hologram is talking to Jabba, the film has been reversed. The camera shows Jabba sucking on his 'bong' and then pans down to Salacious Crumb. However, during the Jabba shot, the air bubbles in the 'bong' are going down, as opposed to surfacing. Behind Jabba, you can see the smoke created is also going downward.
Revealing mistake: In the shot where Vader throws the Emperor into the pit, the Emperor is obviously a puppet, because he doesn't move at all. (01:52:15)
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.
Revealing mistake: If you watch closely (or play it in slow motion) when the scout trooper punches Han, you can see that the troopers hand never actually comes into contact with is face. (00:55:45)
Revealing mistake: In the scene where Han and Chewie sneak up on the two storm troopers on the Endor moon, Han steps on a twig, it snaps, and the trooper turns around and punches him. Watch in slow motion as the punch is released, Han starts falling backwards before the fist hits him.