Continuity: In the scene where 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.

Continuity: During the alien chest-burster/death of Kane sequence, Kane's shirt switches back and forth from being tucked-in, to being un-tucked throughout the scene.

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.

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.

Continuity: 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.