Continuity mistake: When Ripley is in Mother, you can almost read the words on her face as if a small projector is beaming these words on her. The screen she is looking at has 4 lines of green text. The image on her face is 1 line of white text. (01:19:15)
Continuity mistake: In the escape pod Ripley presses buttons to release gases onto the alien. Watch her helmet between the cuts. During the first two gas releases the inside of her helmet is foggy and streaked; cut to the alien reacting to the third gas and when the camera returns to Ripley the inside of her helmet is clear.
Continuity mistake: It can be seen multiple times throughout the film towards the end that there is no cat in the box. This is best seen when the front or back of the box is toward the screen when Ripley is moving down a corridor. (01:36:40)
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.
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.
Continuity mistake: When Ripley sits in the escape pod, she secures her seat belts. As she flushes the alien the belts are on then off then on again.
Continuity mistake: After Ash is decapitated, Ripley says they are going to blow up the ship. In the shot where Parker looks back, Ash's head is slanted to the side. The shot cuts back to Parker where he turns on the flamethrower and incinerates Ash, whose head is suddenly flat on the table. (01:25:40)
Continuity mistake: As soon as Ripley finishes her final conversation with decapitated Ash, she unplugs him and says "We're gonna blow up the ship." A second before she says this, Ash's eyes are black and completely different to what colour they were before. (01:25:15)
Continuity mistake: When Ripley is trying to re-activate Ash, Ripley flattens his head onto the ground. Her left hand is placed on Ash's head and her right hand is placed on the skin below Ash's neck. As soon as Ripley gets the head fixed on the ground, she begins to remove her right hand from the skin below Ash's neck. In the instant shot following, her hand is suddenly back on the skin below Ash's neck. (01:23:05)
Continuity mistake: After the Alien bursts out of Kane's chest, Parker grabs a knife and goes to stab it, and Ash raises his arm at him, telling him not to. The alien then runs off and visible in the back of shot is Ash still pointing at Parker with his arm raised. Then in a sudden close-up of Ash, his arm is now lowered. Although, his entire arm isn't visible, just under his shoulder, the top of his arm is still visible, so his arm is definitely not in the same position as it was in the previous shot. (00:57:30)
Continuity mistake: When Dallas is in the air vent trying to flush the Alien out of the ship he is carring a flamethrower. Right before his demise, he is told by Lambert to "Get out of there" or something to that effect. In the next shot you see him descending a ladder clearly WITHOUT the flamethrower. However, in the shot after that (when he is lowering himself to the bottom of the ladder, right before the alien gets him) he somehow has the flamethrower again (it even got reslung around his shoulder). (01:15:25)
Continuity mistake: Near the start, when Dallas is at the entrance to Mother, one of the lights to his left, the fifth one down from the ceiling, is not lit. The next shot from inside Mother looking out shows that light is now lit. This exact same thing happens a second time when Ripley is entering mother later in the movie. These two scenes must have been shot at the same time. (00:08:20 - 01:18:35)
Suggested correction: That light totally had enough time to light between the frames, because it was hidden by the sliding door for a brief moment. Lights were lit one at the time, not all at once, so this one starts to shine just a little bit later than all the others, that's it.
I consider this a valid mistake and the correction is a stretch. The lights only took about 2 seconds to light up, but she's at the door for almost 3.5 seconds after they're on and the last light did nothing. Then in the half second it took the door to open, the last light is now on.
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: When Ripley runs back into the control room to abort the self-destruct, one shot shows the timer reading 5 minutes. The shot then cuts to Ripley pulling a lever back to where it originally was and yells out "Mother, I've turned the cooling unit back on! Mother!" The shot then cuts back to the timer where it can be seen it has only gone down by 2-3 seconds, but all of what just happened took longer than that. (01:38:40)
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)
