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.
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.
Continuity mistake: In the sequence when the APC is fleeing the hive, and the alien punches a hole though the windshield to get to Ripley. The hole disappears in the next shot when the light reflects off of the glass.
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.
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 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.
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: 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.
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.
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.
Continuity mistake: As the marines first enter the med labs, Drake at the front, there are some neon tubes hanging from the ceiling that slowly rotate to face the camera. As the angle changes and Drake goes round a corner, the neon tubes rotate to face where the camera previously was again in the background.
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 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: 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: 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.
Continuity mistake: When Ripley and the marines go to where the colonists are, there is a ladder in front of the vehicle that was driven inside; the ladder would be in the way if the vehicle was driven further in. But when all hell breaks loose and Ripley decides to drive the vehicle further in, to rescue the marines, the ladder is gone - in fact, a lot of things look different the second time round.
Continuity mistake: Just before the marines are extracted, Hicks tries to close the APC door. An alien stops the door and tries to get inside. Hicks picks up his shotgun and jams the muzzle into the alien's mouth and fires, killing the alien. The alien falls away, they immediately close the door and the APC begins to back up. The camera shot changes to an outside view of the APC. If you look at the area as the APC backs up, you'll see falling debris but no dead alien. It's doubtful the marines would've pulled it into the APC as we don't see it again, or that they would've kicked the corpse out really far due to the acid.
Visible crew/equipment: When an alien rises out of the water to kidnap Newt, you can see two wires lifting its tail out of the water.
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 doing his knife trick on Hudson, his hair changes significantly from one shot to another, especially on the top and around the ears. In addition, at the end of the scene, Hudson is given a meal that was taken off a heater nearby on the table. That meal was apparently not placed back for some of the closeup shots, as it is missing from the heater. Finally, the time shown on Bishop's wristwatch seems to change radically about half-way through the scene.
Continuity mistake: Hicks gives Ripley a wristwatch which is linked to his tracker. If you look at the watch, its face is mostly black with a tiny portion that is clear. However, when Newt has the watch and falls down the tunnel and is wandering around the pool of water before Hicks and Ripley find her, you can see that the watch face has no black - it has a full clear face and appears to be a standard digital watch of the 1980's era.
Continuity mistake: When Ripley is driving the APC, the passenger side front tire is on fire and engulfed in flames. When she runs over the alien with the same tire, the flames are completely gone. In the next shot and several shots after, the tire is completely engulfed in flames again.
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: As Ripley tucks Newt into bed for a nap, the angle of the reading lamp over the bed changes about 45 degrees following their close up discussion of the scary dreams of the doll.
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.
Continuity mistake: When Ripley and Hicks are discussing their defence plans after the first encounter, they decide to put one of the auto guns in each of three locations, leaving one left over. When we see the guns in place they have actually used two guns in each of two locations, that is, two pairs.
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.
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.
Continuity mistake: In the extended edition of the film, Ripley, Gorman, Burke, and Bishop meet up with Hicks at the entranceway. It is raining heavily and Ripley's hair gets soaked. But when she is inside a few minutes later, it looks as if she has had time to dry her hair thoroughly between now and then.
Continuity mistake: As the ship carrying the marines to the planet where communication has been down for too long, the pilot of the ship says, "Where's the damn beacon? Oh, I see it." But when she says the last bit, there is no vision of the beacon; it only comes into sight a few seconds later.
Continuity mistake: When Newt is caged in the resin, the cloud of steam around the egg completely disappears when Newt is seen from the p.o.v. of the egg.
Revealing mistake: Briefly after landing on the planet, we see the APC trundle across the ground. The way it moves shows it is a model, every little bump makes it jump up and down. This is a shame, considering that when it is inside the facility and in subsequent shots, it looks very good.
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.]