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: In the shot of the alien's tail wrapping around the back of Lambert's legs right before it kills her, the floor is different to what it was before, there is water suddenly falling from the ceiling when there wasn't before and her trousers have changed from white to blue. This is because that shot was originally intended to be used when Brett was killed.
Continuity mistake: When Dallas uses the pen to catch the acid, it melts the back of the pen to the point that it's deformed and smoking. When he hands the pen back to Brett, the back of the pen is no longer deformed at all but it's still smoking.
Continuity mistake: When the dead facehugger falls onto Ripley in the medical room, it lands on the floor with its legs up. The distant shot shows its tail has looped under its body, but the close-up shows the tail entirely straight.
Continuity mistake: When Ripley backs into the shuttle closet you can see three axes on the wall next to her, when she starts to get into the space suit there are only two, then when she puts the helmet on there is only one.
Continuity mistake: When the crew is chiding Parker for Brett saying nothing but 'Right' all the time, Brett's cigarette is first short, then long, then short again.
Continuity mistake: When Dallas, Lambert and Kane first discover the Space Jockey, the room is so well illuminated you can see the Jockey, its cannon thing and the floor around it perfectly. In all close shots, it's suddenly pitch black and all we can see is the small areas lit by the crew's helmet lamps.
Continuity mistake: In the air duct when Dallas touches the slime, the amount of slime on his hand changes between shots.
Continuity mistake: When Dallas, Kane and Lambert are exploring the alien spacecraft, you can see that there are wipe marks inside of their helmets that change between shots, most notably on Kane. This is because the helmets were built with out sufficient exhaust systems for the actors. Of course, the characters wouldn't be able to wipe the inside of their helmets with them on.
Continuity mistake: When Ash calls Dallas to the infirmary, Dallas appears in a white shirt that has a blue patch or tag above his left/front pocket. Then he and Ripley leave the infirmary and have an argument in a corridor. Suddenly, Dallas' shirt has a gold patch where the blue one was moments earlier.
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: 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: 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.
