Visible crew/equipment: Just after Ripley drops the first flare (on her way back to rescue Newt), the shadow of a crewmember is cast on Ripley's back.
Continuity mistake: After the Alien Queen is sucked out of the airlock, Ripley climbs back onto the platform and closes the doors. In a shot from above, her left leg has not yet crossed the mouth of the opening. Cut to a closer shot and half her body is already on the platform.
Other mistake: When Hudson is looking for bios on the screen, it shows them on level 3. But when Frost falls down the hole after being hit with a flamethrower, he falls down a number of levels.
Continuity mistake: When Ripley loads the magazine into the gun as she prepares to rescue Newt it says 95 rounds but when she is going down the elevator shaft it says 42 without her firing a shot.
Revealing mistake: The "mission time" clocks on the video screens are very inconsistent most of the time they're seen. At several points they jump back and forth from 00:01:31 to 00:01:32 then back again even though all of them are supposed to be synchronized.
Continuity mistake: When the team approaches the colony entrance to open the outer door, all the marines are inundated with pouring rain. Yet the moment they enter the compound and start searching, they are all dry.
Other mistake: They should have had much more than only twenty-six minutes to save Newt. Bishop said that the reactor would blow in four hours after seeing the emergency venting. He said it would take forty minutes to crawl down that pipe, an hour to patch into and align the antenna, thirty minutes to prep the ship, and fifty minutes flight time. 40+60+30+50=180 minutes, gives them an hour spare.
Suggested correction: And how are you so sure everything went smoothly without problems? Any of those things could have taken longer, leaving only 26 minutes.
Other mistake: The POV shot when the alien kills Burke is reused minutes later as the shot of the alien attacking Gorman and Vasquez as the grenade goes off.
Revealing mistake: When the swarm of aliens drops through the ceiling panels, you can tell that Ripley and the marines don't have magazines in their rifles. It's especially visible when Ripley looks up before Hicks raises the panel and sees the aliens.
Visible crew/equipment: When Ripley is first shown in the loader after asking Apone if she can help, after the loaders claws grab the crate and she swings the crate around and asks "where you want it?" you can see tubes trailing off to the left of the loader that don't connect to anything in frame. The tubes are on the right side of the frame just under the crate in the loader's arms.
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 the crew is having breakfast the first time, at the end of the table is a non-moving kinetic orbit toy in 2 shots, then in the 3rd shot it's moving with no one touching it.
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: 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: In the beginning, Ripley wakes up in a sweat, practically crying. She rings Burke on the visual phone and she has a droplet of sweat on her nostril, but it's there, then it isn't, then it, is; it comes and goes.
Continuity mistake: When Ripley pulls Newt out of the cocoon, she has cocoon stuff all over her back, but a second later it's gone.
Continuity mistake: In the beginning, when they find the colonists, they have IDs under their skins, but on the read-out there's only about 6 of them, when there should be about 100 of them. Later, on the read-out, we see the right amount of IDs.
Visible crew/equipment: During the first attack scene in the hive, a marine gets killed and as he falls between the stairwells, his flamethrower goes off. Just as he falls down into the hold, you can see a crew member's foot moving backwards out of the shot.
