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: 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)
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: 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.
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)
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: There are 2 squads of marines in the film, 9 in total. (Not including Gorman or Bishop, and also not including Ferro and Spunkmeyer who stay in the dropship and do not enter the compound.) Squad one is Apone, Vasquez, Hudson, Crowe, Wierzbowski. Squad 2 is Hicks, Frost, Dietrich, Drake. But as the marines enter the compound you clearly see 5 marines running up to the door, and shortly after, Apone shouts for second squad to move up. As they move look closely and you will see 5 more moving up. 5+5=10, who is the mystery marine?
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.
Factual error: The marines' rifles are supposed to fire "standard armor-piercing rounds", which should have done way more damage than producing a few sparks on the frail sheet metal table in the med bay when Hudson shot the face-hugger threatening Newt. "Standard AP rounds" would have pulverized the table and created a huge hole in the wall behind it.
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: 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.