Continuity mistake: Right after Ripley is rescued from the Narcissus, look at her hair in the clinic. At first there is a wide angle shot of the nurse coming over to her with some small talk. Then, when the shot changes to a close up of Ripley and the nurse, her hair has puffed out about 2 inches more. (00:05:40)
Continuity mistake: When Gorman and Burke come to see Ripley to persuade her to come to LV-426, Burke hands her a plastic card so she can call him if she changes her mind. At this point Ripley lights a cigarette. The scene then cuts to a view of the whole room, and Ripley can be seen lighting the cigarette again. (00:15:15)
Continuity mistake: From Director's Cut. The scene where Al is being quizzed about the ownership of the discovery of the object discovered by the "Mom and Pop" survey team tells us that a call has been put in by that team to the base. But then when they discover the ship, where is the call? They explicitly do not call it in. Now this is not just a general enquiry because they were sent out there two weeks ago to a specific grid reference. "He says he's onto something." (00:16:34 - 00:18:38)
Continuity mistake: Director's Cut: During the briefing scene on board of the Sulaco, Lt. Gorman begins the briefing. He gets interrupted by Hudson. Before the shot changes to Hudson, you can see Gorman standing straight in front of the marines and looking straight ahead. In the shot from Hudson, you can see Gorman looking to the right to face Hudson. He still should be looking straight ahead. (00:32:00)
Continuity mistake: When Ripley is giving Newt the cup of hot chocolate the handle is on Newt's right. When it shows the close up shots, the handle is on Newt's left. (00:48:40)
Continuity mistake: When the Marines first enter the reactor room, the "Mission Time" readout on the shoulder mounted cameras jumps backwards and forwards in time with every cut. (00:54:10)
Continuity mistake: When the marines first encounter the Alien hive in the reactor, and Ghorman asks Hudson what he's seeing, we're treated to a shot of Vasquez' helmet cam, and her name in the bottom right is missing her first initial. [Timecodes are from the special edition of the movie. The first one is the one for the error, and the second is a different scene with the initial present for comparison.] (01:04:00 - 01:20:00)
Continuity mistake: In the sequence when the APC is fleeing the hive, and the alien punches a hole through the windshield to get to Ripley. The hole disappears in the next shot when the light reflects off of the glass. (01:07:05)
Continuity mistake: In the APC, Ripley puts on a headset to talk to Hudson. When the fighting is underway, she turns to Gorman to "get them out of there." Headset's gone. Next cut to, headset's back for Gordan to pull it off of her. (01:15:30)
Continuity mistake: In the shot were Ripley is driving the crew away from the aliens, note the big dent in the vehicle. Then in another shot when they finally get outside, there's a shot from the side and the dent is gone. (01:19:15)
Continuity mistake: During the scene where Ripley is trying to break the window with a chair, there is a scuff mark on it before she hits it. It goes away and comes back. (01:32:15)
Suggested correction: The view from Ripley's side is looking up at the window and from the other side level where there are reflections at the same height (using Ripley as a reference) of the scuffs. There is no way to tell if the scuffs are there or not.
The Director's Cut (and maybe even on the theatrical version) on the 2010 Blu-ray removes the scratches, so I assume they were not supposed to be there initially.
Continuity mistake: Towards the end of the film where Ripley and Bishop are in the dropship flying towards the atmosphere processors, Ripley begins to build a composite weapon from a flamethrower and a pulse rifle. She pulls a flamethrower from a rack on the wall, but lays pulse rifle on the deck of the ship, and then pulls a pulse rifle from the rack and lays a flamethrower on top of the pulse rifle. (01:49:05)
Continuity mistake: You may need slo-mo to see this: towards the end of the film, when Ripley is going down in the elevator to save Newt, pay close attention to the indicator screen counting off the floor numbers as they pass by. It goes floor 8, 7, 6 then 7 again and then 5. (01:51:20)
Continuity mistake: When Ripley enters the hive, right before she lights the second flare, she fires a flamethrower burst. When she does, the starter flame goes out, then in the next scene it is on again. (01:53:50)
Continuity mistake: In the end of the film, when Ripley pulls the Alien queen down in to the hatch we can see that the bulb on top of the machine cracks but when they are lying on the hatch the bulb is whole. (02:05:10)
Continuity mistake: When Ripley and the Queen fall into the airlock, look closely at the Queen's tail. It seems that the sharp end of it is broken off, or missing. Later on in that scene it is back. (02:10:25 - 02:11:25)
Continuity mistake: When Bishop is impaled by the Queen alien he spews android fluid/juice all over his chin. In a subsequent shot his chin is clean and he is spewing fluid again. The following shot his chin is once again covered. (02:23:30)
Continuity mistake: Director's cut: When the autoguns were mowing down the army of Aliens coming through the service tunnels, there should have been enough acid blood being sprayed out to melt a massive hole in the tunnel. But the acid has no effect at all on the tunnel walls.
Continuity mistake: There are many holes in the 'squad structure' once the marines arrive at Hadley's Hope. Gorman calls up 1st Squad (comprised of Apone, Vasquez, Crowe and Hudson) to approach the colony first. They do. But, if you look closely, there are other marines present. When they arrive at the door Apone goes straight to the console, Hudson holds back to our left and Crowe rests in the background on the right of the screen. Apone then calls Hudson up to do a bypass. If you look closely as Hudson runs up to the console you will see Frost in the background, he steps up to the left of Crowe (his left) and on the left of the screen, you will see Wierzbowski crouch with his flamethrower, slightly obscured by Vasquez (look between her legs). Then 2nd squad is called up. It is comprised of Hicks, Dietrich, Drake, Frost and Wierzbowski. Watch the shot of Vasquez entering the lock and you will see Frost standing roughly where he was when the 1st Squad ran up. Clearly the original scene had all the marines descend on the North Lock simultaneously. The confusion continues when Gorman, Ripley, Burke and Bishop enter the South Lock. They meet up with Hicks' squad yet Dietrich and Wierzbowski are curiously absent.
Continuity mistake: When remote sentries A and B start firing at the aliens in the tunnel, the display on A shows 500 rounds before they start firing. However, both remotes A and B had been previously test-fired by Vasquez and should show lower counts.
