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.
Continuity mistake: When Christine is fixing herself after the guys smashed her up, her front end is shown to be perfectly straight for its entire width and the hood is missing. Near the end of the scene, the front end is shown crushed and unfolds again, and the hood appears from nowhere.
Continuity mistake: Near the end of the movie, the panel has JPE 1704 TKS showing, but when Joshua finds the launch code, CPE 1704 TKS is blinking on the screen.
Visible crew/equipment: About one hour and fifty minutes in, the two stars get into a fight and are rolling around the floor. In at least one scene, a cameraman, with a handheld Panavision camera, is quite visible in the middle of the shot.
Visible crew/equipment: At one point Clint Eastwood is out in the woods practising his shooting with his new .44 auto-mag pistol. While he's doing this, a car pulls up and a black male begins to get out with a shotgun. (We later learn that this is his partner). As the man with the shotgun is opening his door, if you look at the reflection in the car window, you can clearly see the reflection of a man with a camera and another man standing next to him.
Visible crew/equipment: After Calvin Bouchard makes the announcement to tourists visiting the Undersea Kingdom to evacuate, we see the tour guide lead the crowd through the tunnels. The green screen backdrop is still in the final cut. They forgot to add in the underwater effect.
Visible crew/equipment: When Dr. Raymond leaves with Mary in his car after talking to Norman, when he drives off the entire crew and equipment can be seen reflected in the side of his car. (00:41:50)
Visible crew/equipment: When the telephone rings, Cujo bounds over and begins barking at it. After he has finished barking, he lingers in the window frame, and then, you can see a crewmember's hand come in, and pull him down from the ledge. (00:55:30)
Continuity mistake: In the morgue, the trays containing medical tools change positions between shots. They are strategically moved further up the bodies for the closeup to conceal the genital area, whereas before they only covered the legs. (00:06:30 - 00:07:40)
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)
Continuity mistake: At the end, when the naked killer is kicking the door, there is blood on the door from his feet. Several shots later, the blood on the door changes even though he has stopped kicking it. (01:34:45)
Continuity mistake: One of the last scenes of the movie, when Karen Silkwood is driving away from Dru they show her sitting in the car waving at him and the rearview mirror is clearly visible. Then she starts driving away, and the rearview mirror is gone. Then as she drives off, you can see the mirror has been replaced.
Revealing mistake: The outline of the blood pack is visible under John's shirt when Stillson's body guard shoots him. (01:37:35)
Continuity mistake: The rear view mirror on Solo's convertible is there in wide shots, but missing in all the closer shots throughout the Vegas car chase scene. (00:21:00)
Visible crew/equipment: In one of the first scenes, two plain-clothes cops are chasing a suspect. The suspect runs through a house and down some stairs. Look under the stairs. You can see a person trying to hide in the dark under the stairs, presumably a stage hand.
Factual error: When the Brainstorm project is taken over by the sinister military operative Landan Marks, he begins weaponizing the brain-interface technology and testing it on the human guinea pig Gordon Forbes. As Gordon is subjected to increasingly difficult fighter-jet simulations, Landan Marks gleefully exclaims to military observers, "Now watch this! He can take a full 10-G rollout without losing control, just by thinking about it!" In the flight-simulator cockpit, Gordon grimaces, but the Brainstorm device allows him to remain conscious and maintain control despite his physical distress. But the fact is that no flight simulator in the 1980s or even today would be able to simulate extreme G-forces as described in this film. In fact, flight simulators then and now can't approximate even low G-forces. Only a giant centrifuge can produce such forces; but Gordon is not in a centrifuge for this scene. It's simply a flight simulator.
Continuity mistake: In the airplane, the window shade in the seat next to the main character repeatedly opens and closes between shots.
Other mistake: In the scene where Jesse enters Monica's apartment, he consults her calendar to see in which class she is in school that day. There's nothing unusual about this until you look closely at the calendar and see that time time is early/mid August. Did her school's academic year run differently from other schools' years, or was she taking summer classes? The latter doesn't seem likely.
Continuity mistake: When Jeremy Irons is talking to Patricia Hodge in a bar in the movie's first scene, the beer he is drinking changes levels from full to half full to full again.