Continuity mistake: When Lando says "Go on, you pirate" his entire outfit is reversed - his shoulder holster strap goes the other way, and his rank insignia changes side too. (00:50:00)
Revealing mistake: After the heroes get snared and surrounded by Ewoks on Endor, the Ewoks mistake C3PO for a god and start bowing. There is one Ewok which only has holes where the eyes and mouth are and you can see the eyes and mouth of the person inside the Ewok costume. (01:07:50)
Continuity mistake: The shots at the end of the movie with the emperor are obviously lit from the front. This exposed too much of the emperor's head inside the cowl so it was darkened to make him look more menacing. Look for the added shadow - it looks like someone took a black magic marker and coloured on the film. (This may be corrected on the re-release).
Character mistake: As they take Han away, watch Lando. Just before he tilts his mask down so we can see who he is, he whacks his head on the doorway. (00:16:20)
Continuity mistake: When Luke is prodded out to 'walk the plank', into the Sarlaac pit, he jumps off the board and springs himself back up. If you watch this carefully, you can see that only one hand catches the board and that just by the fingertips, but in the next shot both hands are springing him upwards. (00:31:20)
Continuity mistake: When Leia gets shoved on Jabba's belly upon her capture, watch carefully - in the side shot of her being pressed against him, we see her arching back from her master-to-be as he talks. Their faces are a good foot or so apart. However, in the next shot of her, we see her face is mere inches away from him as he licks his lips in horny anticipation. (00:19:54)
Visible crew/equipment: When Han is running from the explosion of the shield generator on Endor, you can see his reflection in the plexiglass that is used to protect the camera from the explosion. Since the reflection isn't of Han's back, the plexiglas must be in front of him. (01:49:55)
Continuity mistake: When the guards take Chewie to his cell, you can plainly see that when the guard closes the door you only hear it close. You can see that it is still open. (00:17:15)
Continuity mistake: When Lando is hanging over the pit, he has gloves on, then he has no gloves, then they're back and so on.
Continuity mistake: When Jabba pulls Leia's chain, there is a long shot as she leaps up onto his throne, and he is holding his wine goblet in his left hand, and pulling her chain with his right. However, in the very next shot (in which he says "Soon you will learn to appreciate me"), the entire frame is reversed. This is evident in Jabba's holding the wine in his "right" hand, his higher-placed nostril (which is normally on the left on his face) is now on the "right," Leia's reversal of her armlet, etc. (00:29:20)
Continuity mistake: When Vader is driven to his knees during the duel, his elbow is over the handrail, but when Luke cuts off his hand, Vader's wrist is lying over the handrail instead.
Continuity mistake: When Vader ignites Luke's lightsaber, Luke is standing next to a column. When Vader extinguishes the lightsaber, Luke is standing further away from the column. In the next shot, he is standing next to it again. This can only be seen in the widescreen version. (01:21:10)
Revealing mistake: In many far away shots of the sail barge you can see it is on a dune rather than in mid air.