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: 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)
Continuity mistake: When the search party goes into the boneship, they are wearing full space suits, and hoods under their helmets. When Kane's helmet is first cut off, he has no hood on. (00:36:45)
Continuity mistake: After the alien leaves Kane's face and the rest of the crew are looking for it in the infirmary, there is one shot that bears re-watching. When Ripley is looking upwards over the prone Kane, you begin to see the dangling tentacles of the alien in the background. At the same time, the collar on her outfit begins to pull downward, un-naturally, until the alien drops onto her shoulder. I believe there was some sort of string or rope fed from below, through her clothing, and was attached to the alien and was pulled to have the creature drop onto her. (00:48:33)
Continuity mistake: While the Alien is bursting out of Kane's chest, notice how the blood stain on his shirt changes in size from a large stain to a small one and finally back to a large one. (00:56:00)
Continuity mistake: During the alien chestburster/death of Kane sequence, Kane's shirt switches back and forth from being tucked-in, to being un-tucked throughout the scene. (00:56:15)
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: 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: When Ash has gone crazy and is attacking Ripley, his white android juice/sweat/fluid is flowing from his forehead. After he throws Ripley into the room with the nude pictures on the wall, there is a new shot. In it, Ash is walking into the room, but there is no android fluid on his forehead. (01:21:35)
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: While Parker is decapitating Ash with the fire extinguisher his T-shirt is not torn, just after he says "Ash is a goddamn robot" his T-shirt is now torn and held together with a safety pin. (01:23:20)
Continuity mistake: When Lambert deactivates Ash by using a cattleprod, you will notice that she is wearing brown cowboy boots. Minutes later as the Alien goes to wrap its tail round the back of her legs, she is now wearing a sneaker - though you only see the toe cap it is clearly not a brown cowboy boot. (01:23:55 - 01:33:40)
Continuity mistake: When Ripley reconnects the dismembered Ash droid, the switch from dummy to actor is blatantly visible. A few minutes later the android is incinerated, at which point the dummy head has reappeared and the android's left arm has disappeared. (01:24:39)
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: 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: Before Lambert is killed, the alien twists its tendrils around her leg, and we see that she has sneakers on. However, when Ripley notices Lambert's dead body (hanging?) she is barefoot. [The rest of the scene was cut. According to the novelization, Lambert was actually intended to be shown completely naked and sexually ravaged. Director Ridley Scott had toyed with the idea of creating the illusion of the Alien having raped her before her death. He thought better of it and cut the scene. Still a mistake, but there's why.] (01:33:40 - 01:34:25)
Continuity mistake: When Ripley fails to override the self-destruct system she hits the computer with her flamethrower. As this happens it cuts to the screen which displays 10 minutes left until detonation, when really it's under 5. (01:35:15)
Continuity mistake: In regards to the self destruct mechanisim of the Nostromo, It is said that the time from the setting to detonation will be 10 minutes. This is also confirmed by the voice of "Mother" who says, right after Ripley initiates the sequence, that the detonation will take place in "T-Minus 10 minutes" However, in the next shot you see one of the monitors which shows a countdown with 10 minutes and an extra 30+ seconds on it. (01:35:55)
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)
