Revealing mistake: When Ripley first burns the egg plants inside the nest, the Queen screams and moves her head. The steel rod used to hold the Queen's head to the rest of her body is easily visible.
Revealing mistake: When Newt is sliding across the grated floor, Bishop reaches out to save her and you can clearly see he's standing in a hole to make him look cut in half. (02:11:40)
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: 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)
Other mistake: When the autoguns fire at the aliens for the second time, there is a flashcut of an alien being blasted apart. Watch closely; it's the same clip of the alien that Vasquez shot and subsequently sprayed Drake with acid. (01:18:00)
Revealing mistake: Right before Ripley discovers all of the eggs that the Alien Queen has laid, look at Newt's fingers as Ripley is carrying her - It's obviously a fake hand. Newt is in fact a dummy in this scene, and others preceding the chase from the Alien Queen. This dummy was created so that Weaver would not get exhausted carrying a real child around for so many scenes/takes.
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: 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: 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)
Revealing mistake: In the Special Edition of the film, in the scene where the Marines are conducting their initial search of the colony, the shot where Hudson and Vasquez enter the "negative situation" room (where they find the hamster) is obviously sped up. You can tell by how quickly Vasquez seems to move when she comes through the door.
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: 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: 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.
Continuity mistake: In the scene close to the end of the film when Ripley is getting the weapons ready in the 2nd drop ship to rescue Newt, she slams in a new ammo clip into the pulse rifle and the counter reads 95 rounds. Moments later when she is in the elevator descending into the aliens lair the counter is reading 42 rounds, even though she hasn't fired a shot.
Continuity mistake: During the Cargo Loader vs. Queen fight, Ripley grabs the Queen and it closes around her neck. In the next shot it is closed around its head. It changes extremely quickly; therefore it couldn't have changed position.
Revealing mistake: When Ripley rescues Newt, an alien comes from a corner and it jumps from side to side. In the second jump, you can see two wires on its back. (01:50:30)
Continuity mistake: At the beginning of Aliens, when the door is cut through, the markings and lettering on the transparent cover of Ripley's suspended animation pod are different from those that were on it when she was asleep at the end of Alien.
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)
Visible crew/equipment: In the Reactor Room scene, where the Marines first encounter the aliens, there's a part where Dietrich is grabbed from behind and accidentally ignites Frost with her flamethrower, causing him to fall down the stairwell. As he's falling, if you look, you can see part of a leg and a cable belonging to one of the technicians on the set. (01:14:05)
