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: In the scene where Bond is trying to escape with his new Mini-Jet, he flies through a big hall. If you look carefully, you can see that the aeroplane which is supposed to be flying with its wings vertical is fastened to some kind of sustainer.
Revealing mistake: At the beginning of the film, Bond is on a training exercise to free a trapped hostage. Since it is only a training exercise, everyone involved is using blanks in their weapons. Why is it that when Bond fires an automatic rifle, we hear multiple bullets ricocheting? (00:03:25)
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: Right after a fleeing felon is shot dead and falls into the pool (whereupon Murphy says "He's bought it"), he appears to be swimming to stay afloat. (00:12:10)
Revealing mistake: As Harry approaches the Acorn Cafe (which he is about to shoot up), you can see that the two window panes just right of the door are candy glass. This is confirmed when one of the 'heavies' goes flying through them after Harry gives him the .44 Magnum treatment.
Revealing mistake: Solo's stunt double is easy to spot in the scene where he rappels down the cliff and jumps into the truck bed. The double has quite a bit more hair. (01:12:30)
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)
Revealing mistake: For the first kill, Warren is chasing the naked girl while being equally naked. And yet, in the side view of his run in the woods, Gene Davis' pants pop in frame a couple times. (00:08:00)
Revealing mistake: A woman cuts her finger on a miniature ninja doll sword, but she only touches the blunt back of it. (00:18:45)