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 David starts the game with Joshua, it asks him what side he wants to be (1 for the U.S. and 2 for USSR). Look carefully at the closeup of the computer screen - his choice, 2, appears long before David actually presses the key.
Revealing mistake: In the scene when Christine smashed repeatedly into the silver Camaro at the gas station, there is no motor in the Camaro. The hood of the Camaro gets knocked off the hinges and is twisted sideways on the car. As Christine is in reverse and dragging the Camaro along you can see down through the hood that the engine compartment is empty. Minus a transmission also, I believe. (01:15:30)
Revealing mistake: At the end when Norman is being stabbed at by Mary dressed up as Mother, Mary stabs Norman in the hand. You can plainly tell that this hand is a doll, as it is not a natural colour, it is stiff and is stationary, unlike Norman who is moving his hands. (01:35:10)
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: The shark attacks a little wooden platform in the water which 3 men then fall through. You can see this is made from a lightweight wood such as balsa so that it will break apart easily, and it's not the sort of wood such a thing should be made of.
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: The outline of the blood pack is visible under John's shirt when Stillson's body guard shoots him. (01:37:35)
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: When Tad begs his Mom to try turning on the car, Tad crosses his fingers before his Mom asks him to. (00:51:15)
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: Obvious dummy when Mick slams into Paco's little brother. (00:21:05)
Revealing mistake: As Car Driver approaches the yellow caution sign of the winding road they are traveling, he continues driving straightforward, never once turning the wheel, and periodically turning his head to talk to Passenger.
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)