Other mistake: During the fight between Luke and Darth Vader, when Vader throws his lightsaber at Luke, it is not only Luke's lightsaber from Episodes IV & V instead of his own, it is ignited upside-down, with the blade protruding from the pommel rather than the emitter. (01:50:20)
Revealing mistake: Vader tosses the Emperor and then collapses on some barrels next to the railing. Look at Vader's arm - the one with the severed hand. His arm is bent at the forearm in a way that arms just don't bend. This is because David Prowse's hand was inside an "elongated" arm to simulate it being chopped off. He couldn't keep his wrist straight, giving the appearance of a bent forearm. (01:57:00)
Revealing mistake: When Jabba's sail barge begins to explode, you can see that one of the floating skiffs is standing on stilts. (00:36:30)
Other mistake: DVD and Blu-ray only: in the Battle of Endor, right after the rebel fighter says "there's too many of them." The Millennium Falcon goes into 1/4 of the picture, you can see a black square spot on the right side of the flash frame due to a bad filter usage. Widescreen only. It does not appear in the 4K release even though it wasn't removed.
Suggested correction: This was not a "mistake". The outlines showing around the beast's leg were due to the limited CGI technology at the time the film was made. With advances in special effects in the following decades, the filmmakers were able to enhance the CGI quality in later releases.
raywest ★
But it is a mistake to show that the Rancor is not real, and the outlines show that. It's certainly not intentional.
lionhead
Not sure what you mean that the rancor is not real. Of course it's not real. The issue is, at that time, it was not technically possible to show the beastie without the lines showing. I classify a mistake as something that was not intentional. In this case, it was, due to the limitations of CGI in the 1980s.
raywest ★