Revealing mistake: Mirrors were used to make the corridors appear longer, early in the film when Brett is complaining to Parker about the way they are treated by the company you can see their reflections in the mirror behind them as they walk down the corridor. It's only for a second as they are moving off camera but you can see them in their white overalls.
Revealing mistake: In a scene near the end, the camera operator appears to bump into Sigourney Weaver from behind, prompting her to momentarily break character. Weaver had been frantically running down a corridor with the camera following closely behind. When she reaches a ladder, the view passes her for a split-second and bounces back a step from an apparent soft collision. Weaver glances back at the camera with a look as if meaning to say, "Are you all right?" but immediately resumes climbing the ladder. (01:39:35)
Other mistake: The voice of Mother saying the self-destruct sequence has been activated is heard before Ripley pushes the red button of the last of the four mechanisms used for that purpose. (01:35:58)
Plot hole: The shuttle "won't take four" crew members. Considering it only has two cryotubes (and hypersleep appears to be necessary for any hope of survival), it doesn't seem to accommodate three people, either. (Admittedly I would still go for it, too, if I were up against a 7-foot-tall acid-blooded alien monster with two sets of jaws, but there doesn't seem to be much difference whether there's three or four people onboard).
Continuity mistake: When the ship warms up before the crew is revived, a computer workplace is shown two times: in the second instance, an empty coffee mug sits before the right side of the screen that wasn't there before.
Other mistake: In the opening bridge sequence when the Nostromo's computer wakes up, text is displayed on the monitor. The scene cuts to a shot of the reflection of the text in a helmet's visor. The reflected text should be reversed, but it is not. (00:02:00)
Continuity mistake: Just after Dallas is taken by the Alien in the air shaft, the crew are round a table discussing what they should do next. Ripley has a visible injury to her right cheek, a round swollen injury. In the next scene the injury has gone. It's possible that it was caused by accident filming the scene when she was attacked by Ash with the rolled up magazine, although that happened later in the film.
Character mistake: While entering the atmosphere of LV-426 when the ships starts to shake in a turbulence, there is a close-up at Ripley's screen showing the terrain and at the bottom of that screen we can see "WEYLAN YUTANI NOSTROMO 180246." The correct name of the Company is WEYLAND YUTANI and Nostromo's registration number is 180924609. (00:16:35)
Character mistake: Ripley identifies herself as 3rd officer. She is actually a 2nd officer.
Revealing mistake: The amber warning lights on the ceiling start flashing before Ripley has activated the self-destruct system.
Continuity mistake: When the ground crew prepares to leave, the captain tells them to take weapons. All three crewman are in suits with side arms on hips. These weapons are never seen the rest of the movie.
Suggested correction: With reference to the side arms mistake the weapons are seen, when Kane goes to look inside the egg before he is attacked he is carrying his side arm and holding it ready.
Continuity mistake: As Mother counts the seconds down from 10, after Ripley safely shoots out into space at the end, suddenly the 8, 7, 6 etc. speeds up.
Continuity mistake: Right at the beginning of the movie we see one side of a planet with a ring going around it pointing toward the bottom of the image, then the camera slowly moves to the other side where the ring is pointing towards the top of the image. This would only be possible if the ring around the planet has a figure 8 shape, however later in the film the ring is revealed as flat.
Continuity mistake: Near the beginning when the Nostromo crew are debating over landing on the planet and Parker is arguing against it. Ash addresses him by revealing the company clause that states any potential alien intelligence must be investigated. He even mouths to Parker "no money." In the best shot Ash is directly behind Parker and not in a position where he could have been directly addressing Parker as the previous editing suggested. (00:12:35)
Revealing mistake: When Dallas is in the air shaft he shoots downwards with his flamethrower, you see the flames that he is shooting from the bottom of the ladder, but you can see a flamethrower shooting upwards from the side of the shaft and it also moves in an arc.
Revealing mistake: You can see the blood was initially thrown on John Hurt instead of coming from underneath his shirt. You can also see the underlying prop wrap that was used for the blood exploding out of him just before you see the alien pop out. (00:56:56)
