Continuity: 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.

Continuity: 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.

Visible crew/equipment: In the scene where Kane is struggling on table shortly before the alien rips through his chest, if you put it in slow motion (it's probably best seen on DVD) just before the first patch of blood appears you will see his t-shirt ride up and you can see something strapped to his chest. This is some sort of pack containing fake blood that discharges on to his t-shirt.

Factual error: When Ripley is talking to "mother" by typing, it types "Emergency command overide 100375". "Override" should be spelt with two R's.

Revealing: while Parker is decapitating Ash with the fire extinguisher his T-shirt is not torn, just after he says "Ash is a goddam robot" his T-shirt is now torn and held together with a safety pin.

Continuity: In the scene where 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 clearly not a brown cowboy boot.

Revealing: When Parker shoots the blowtorch at Ash, Ash's face begins to melt off. When the skin melts off we see, in slo mo, that the dummy they used has a sealed mouth.

Revealing: In the rapid closeup sequence in which the alien kills Parker (Yaphet Kotto) by biting him right in the face, a freeze-frame viewing indicates that, at the point of impact, a honeydew melon was used to absorb the bite. Now a honeydew melon has a white external rind, and Kotto is a black actor. Why couldn't the SFX guys, at least, have painted the melon to match Kotto's complexion? This production error is, well, in your face.

Revealing: Parker had hair. When the alien bit through his head the melon they used did not have hair. Also, the seeds can be seen inside the melon when paused at the right moment.

Visible crew/equipment: When Ripley is heading towards the Shuttle towards the end, just before she finds Jonesy, there is a POV shot where the camera is walking towards a ladder. If you look at the floor you can see the dolly that the camera moves on.

Continuity: In the scene where 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.