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.
Factual error: In the balloon scene, it seems like Q can circle around and land on a spot where he flew past previously. A balloon can only fly in the direction of the wind.
Other mistake: Towards the end of the movie, when Superman drops Gus off at the coal mine, Gus begins talking with the coal miners about him and Superman. He then rips open his shirt (as if he had a Superman costume on underneath). How did Gus know that Clark Kent tore his shirt like that in order to change into Superman, or keeps his suit under his clothes at all? The only people who know this fact are the ones who have seen the transformation from Clark to Superman, i.e. the audience.
Revealing mistake: In the early part of the film, Beverly D'Angelo and Chevy Chase check into the hotel/motel. She goes in to take a shower but he barges in. If you look real closely, she is wearing underpants and is not nude although she is taking a shower.
Continuity mistake: When Tom Selleck rolls his plane upside down to dump the Afghan at several thousand feet altitude, you plainly see stationary tree tops at the bottom of the frame.
Factual error: Chuck Yeager's sidekick, Jack Ridley, appears in many key events which occurred after 1957, including the climactic 1963 test flight of the NF-104 plane that Yeager ejects from and is picked up by Ridley. Jack Ridley actually died in 1957, when the aircraft he was co-piloting crashed into a mountain in Japan.
Factual error: When we first see Fatima Blush, she is waterskiing. She is doing several stunts, which are only possible with one ski, the most notable being skiing backwards with one foot in the air. However, when she releases the tow rope and skis up the ramp into James Bond's arms, she is on two skis. (00:45:55)
Visible crew/equipment: Right at the start of the film as the alien spacecraft is landing on the planet, you can see the scaffolding of the studio at the top of the screen.
Continuity mistake: At the wedding ceremony, Prince Colwyn extinguishes his flame inside the basin of water. However in the next shot his baton is aflame again even though he only just put the fire out.
Factual error: The caption says "twenty years later" before the main part of the movie starts. Now judging by the dates we've seen on-screen so far that would be in the reign of William and Mary, not Queen Anne.
Revealing mistake: When Nicole's bike jumps over the kids lying on the ground there's a ramp beside them disguised as part of the road to allow for the stunt.
Continuity mistake: Hawk gets shot, and the stain is on his upper left chest. A new scene shows the stain on the right chest.