Continuity mistake: Widescreen version: When Darth Vader dies, the chin part of his helmet is missing in the last shot. (02:01:30)
Revealing mistake: When Luke and Darth Vader are fighing in the emperor's throne room, they pass in front of a great window. The stars are blurred and they shake. (01:52:30)
Revealing mistake: When Admiral Piett says "I have orders from the emperor myself," the right edge of the matte can be seen on the right. Theater version only. This has been corrected on the 1997 special edition release.
Visible crew/equipment: (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.
Suggested correction: Just loaded that scene up on Disney Plus. The rancor's puppeteer's arm is actually not visible. The rancor is actually an animatronic.
According to Wookiepedia, the Rancor was a puppet, not an animatronic (a man in a suit was tried, though). For the digital releases, it was recomposited, so the visible arm was probably removed.
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)
Continuity mistake: When Wicket wakes up Leia in the woods, the hooded garment he is wearing changes length between shots.
Revealing mistake: (Original theatrical release) When the imperial officer yells "Freeze", he is transparent and you can see the reactor through him.
Visible crew/equipment: After Luke cuts Vader's hand, there's a shot of the Emperor walking downstairs. The shadow of the crew is visible on the lower left corner of the image. It can't be the shadow of Luke.
Revealing mistake: In many far away shots of the sail barge you can see it is on a dune rather than in mid air.
Continuity mistake: In the Rancor pit, watch as the Rancor starts to pick up the pig guard. The Rancor starts to grab the pig guard with its left hand. But while being eaten the pig guard is held in the Rancor's right hand. (00:25:40)
Continuity mistake: Before Chewbacca and two Ewoks swing on a vine to the roof of one of the AT-ST walkers, in a shot of them running, Chewie's bandolier is on the wrong side.
Continuity mistake: When Boussh (Leia in disguise) is about to enter Jabba's throne room, there's a shot of Jabba where his arm is down by his side. In the next shot, on the left side of the screen, his arm is up by his head. (00:13:50)
Continuity mistake: In the sequence of the Emperor torturing Luke, there are several close-ups of Vader. Look at his helmet - it goes back and forth between being clean and being dusty.
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff, you can see a stick supporting the "floating" skiff he jumps over to. (00:32:15)
Revealing mistake: When Luke is in his X-Wing preparing to make the jump to Dagobah, you can see the red text in the TV-type viewer in the cockpit has been superimposed as it remains stationary relative to the frame even when the cockpit is moving around.
Other mistake: When Vader first arrives at the station, he and the Commander are walking past rows of troops. If you look carefully, you will see a Stormtrooper in the back row with his helmet tipped so far forward it's almost off.
Continuity mistake: The birthmark on Leia's back keeps changing sides during the 'Jabba' scenes. This seems to happen when Jabba's tail changes sides. I guess the model of him only went one way, so they flipped the negative.
Continuity mistake: When Vader's shuttle has landed inside the Death Star, behind the stormtroopers on the left side, there is a large structure. But when Vader and Jerjerrod, the Death Star commander, pass these stormtroopers, the structure is no longer there. (00:03:20 - 00:04:05)
Continuity mistake: On Endor, when the stormtroopers come to take Luke away, the door of the lift is heard opening, the stormtroopers are heard walking, and then they appear behind Luke three seconds after the door opened. But in the next shot, we can see that Luke and Vader are too far away from the lift for the stormtroopers to reach them that quickly.
Continuity mistake: When Luke confronts Jabba the Hutt, Jabba says to Bib Fortuna, "You weak-minded fool." In the next shot, Jabba's tail has switched position. (00:22:55)