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.

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.

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.

Continuity: The amount of Alien slime on Dallas' hand changes between shots.

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: Disappearing Android Juice - 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 this new shot Ash is walking into the room but there is no android juice on his forehead.

Continuity: 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.

Visible crew/equipment: On the DVD version, following the chest-burster scene as the camera pans around the darkened corridors on the Nostromo just prior to Dallas ejecting Kane's body, what appears to be a boom-mike is visible only for approx two seconds to the left of the screen.

Visible crew/equipment: After Ripley fails to shut off the auto destruct sequence, she runs through a corridor filled with gas. As she exits from the gas right before the shot cuts away we see her pass by a full sized light. Beside this light is the clear arm and head of a lighting crew member. (This is acknowledged on the DVD commentary).

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.

Continuity: 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.

Audio problem: When Tom Skerritt first "interfaces" with Mother after being awoken, he says, "Good morning, Mother" - it is not even close to being Tom Skerritt's voice.

Revealing: As Ripley stumbles away from the self-destruct system, The cooling rods are not visible. A few seconds later, in close-up, they can be seen to be above the edge of the compartment they are in.

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.

Revealing: When Ash tries to kill the semi-conscious Ripley by choking her with the rolled-up magazine, she begins to gag before the magazine even touches her lips, even though there is nothing in her mouth or throat yet.

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.