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.
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.
But it is a mistake to show that the Rancor is not real, and the outlines show that. It's certainly not intentional.
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.
Revealing mistake: When Superman gets blown away by the bombs, he is being smashed onto the rocks. You can easily see that the rocks aren't real, but also that when he rises up again, you can see the cardboard rocks dent.
Revealing mistake: When the cat rips the painting, it is already damaged before it touches it.
Revealing mistake: During the Amazon women sequence, when Niki gets pulled underwater you can tell by the movement of the water that it was shot in reverse. (00:49:32)
Revealing mistake: As Mr. Connor flies into the retaining wall after being thrown from the Vietnam setting, you can see the "stone" structure wobble.
Revealing mistake: When Denis runs out into the field after the nuclear attack, she brushes up against a dead cow, and the cow moves like a stuffed prop. Real cows are much heavier than that.
Revealing mistake: During the brawl in the 'saloon', the protagonist frees himself from the predicament that had him with a knife pointed at his throat. Cut to one of his friends who punches away one baddie, parries a strike and then "punches" someone else. Sarcastic quotes are needed here, because the straight punch misses its target. (00:44:35)