When Ripley tries to break the glass in the med lab there is a scuff mark on the glass before her first swing. After this finishes, the mark disappears and we see Ripley actually making the mark with her second swing. (This goof was digitally corrected for the blu-ray edition)
(at around 1h 45 mins) In the elevator escape scene where Hicks gets acid sprayed, he holds his gun in front of his face and turns his head to the left. Any acid burns he got would have been to the right side of his face. As they fight to remove his melting chest armor, there are no acid marks on his face. When they leave the elevator, the acid burns are on the left side of his head, instead of the right side.
In Alien (1979), the computer displays have an old-fashioned "teletype" clicking noise when they show data on their screens. However, when the screen in the Nostromo shuttle comes to life at the beginning of Aliens, the display looks and sounds more modern like the other computer screens seen later in the film - even though 57 years have passed and the equipment in the shuttle has not been touched all that time.
[2:10:00] After the Queen falls down the airlock, the sharp end of her tail is missing (obviously unintentional, and overlooked or not repaired by the crew). The tip is back on moments later as she's tumbling through space.
After an alien smashes a hole through the triangular windshield on the APC during the escape from under the cooling tower the windshield is then shown with an unbroken windshield as it exits on to the planet surface.
When the dropship is released, it is shown to be falling immediately as if in full gravity. However, since the Sulaco is placed on a stationary orbit, the dropship should remain in place upon releasing the clamps. An additional burn would be necessary to initiate deorbiting.
The reason given for removing the marine's ammunition is that the facility is a giant fusion reactor and damaging the cooling system will cause a thermonuclear explosion. A fusion reaction does not use or produce fissionable material so it cannot cause a thermonuclear explosion.
The landing sequence is incorrect. The dropship is seen accelerating while deorbiting, such acceleration would send it to a higher orbit. Deorbiting needs deceleration from the orbital speed. The dropship should have fired retro-rockets or it should have turned backwards to use the main thrusters for deceleration.
Moreover, if the Sulaco is exactly above the colony (as stated in the novelization), the dropship should have done a vertical drop (relative to the planet) or should have completed a full orbit while descending to end up right above the colony.
At the end of the movie, the compound explodes and the survivors escape in a shuttle. We see that brightness of the explosion shines on their face. But in fact, the explosion and the light source is behind, so there's no way their sight would be affected by the brightness.
When Ripley is arming up to rescue Newt, she takes grenades from a case that is clearly marked 40mm. The Marines use pulse rifles with 30mm launchers.
When the aliens capture Newt and Ripley tries to go after her, Hicks holds her back and tells Ripley that they can track Newt using the tracking device. Hicks gave the tracker band to Ripley and didn't witness Ripley giving it to Newt so he wouldn't have known this. However, he didn't witness it onscreen; Ripley obviously told him about it offscreen.
[1:29:00]When Ripley goes back to medical to join Newt, she lays her rifle on top of the bed and then lays down on the floor beside Newt. Moments later she looks to break the glass but finds her rifle laying on a table on the other side of the glass. The scene makes it clear that Burke entered while she and Newt were asleep and laid down the opened containment cylinders with the facehuggers inside of them; it's fairly obvious that he moved her rifle out of the room as well (note Ripley's expression when she sees her rifle).
The inquest had gone on for "three and a half hours", yet no one mentioned that LV-426 had been colonized for over twenty years. Given the fact that the board needed to hold Ripley accountable for the willful destruction of the Nostromo for insurance reasons, they obviously felt it was in nobody's best interest to broach the subject.
In the Special Edition, as the Dropship heads to the planet, Cpl. Hicks appears to be asleep. As the ship enters the atmosphere and the marines get ready to deploy, he is clearly awake. In the next shot he is still asleep. Finally Apone says "Somebody wake up Hicks." This is all meant to demonstrate how seasoned a soldier Hicks is: that he can "sleep" through the drop. At no point is he ever actually asleep, he's just very relaxed. He opens and closes his eyes as the conversation interests or disinterests him. Apone is simply making a joke about this.
In Alien (1979), LV-426 is adjacent to a huge planet with Saturn-like rings (which it probably orbits). Such is its size, it can even be viewed from the surface (i.e. when the Nostromo crew investigate the alien ship). The ringed planet is never seen in this film, either from space or from the surface of LV-426 because James Cameron simply chose not to depict it from any angle.
When Bishop saves Newt from being sucked out of the airlock, part of Lance Henriksen's real torso can be seen beneath the false one, emerging from a hole in the floor. (James Cameron acknowledges this error on the DVD commentary; the goof was digitally corrected for the 2010 Blu-ray release.)
[21:00]When Bishop is "doing the thing with knife" the scene is sped up to give the impression that Bishop is lightning fast. Apone can be seen rocking his head back and forth, also at a ridiculous speed.
Gorman states that the M41A Pulse Rifle fires 10mm explosive tipped case-less ammunition, several times throughout the film, you can clearly see casings leaving the ejection port of the Pulse Rifles.
(at around 1h 55 mins) When Ripley turns to find herself in the Queen's chamber, Newt's hand is rigid because she is holding a lightweight Newt dummy.
(at around 4 mins) When the salvage crew shines a light onto Ripley's face while she is sleeping at the beginning of the movie, her right eyelid twitches. Although people's eyelids twitch all the time when they are asleep, Ripley is is frozen in cryogenic suspension - not merely asleep.
(at around 1h 25 mins) After Vasquez cuts a hole in the conduit to allow Bishop to crawl to the transmitter, the metal cut out segment falls into the conduit pipe through the hole. When Vasquez replaces the cut out piece, it does not fall through. This is because she rotated the asymmetrical piece so that it would fit. But if she did not replace it in exactly the same way it was cut, it wouldn't have fit so neatly into the cut-out hole.
Though not mentioned in the film itself, the Blu-ray edition has some in-universe details within the menu screens. One of these details states that the atmosphere processor on LV-426 is 1500 meters high. This is almost a mile, but as seen in the film itself, the processor is nowhere near a mile high.
Sergeant Apone (Al Matthews) can be heard being referred to as "Top" by his Marines. This term is used when referring to Master Sergeants (E-8), but his chevron clearly indicate the Marine rank of Gunnery Sergeant (E-7).
When Bishop goes into the tunnel he says it should take 40 minutes to crawl 188 meters to the transmitter. Even with his limited movement, which didn't appear that restricted it should not take anywhere near that long to crawl 188 meters. However, he may have been using a conservative time frame to give himself some cushion.
As the drop ship is about to be released the window on the right side of the cockpit is not attached to the top of the cockpit.
[21:00]When Newt's mother is calling in the mayday, she is heard saying "Alpha zero two four niner" repeatedly, but the second time she says it her mouth is saying something else. (Special Edition only)
During the ventilation shaft scene towards the end of the movie Vasquez tells Gorman "you always were an asshole". However, you do not see her lips moving at the end after the camera angle changes.
After the first contact with the Aliens at the processing station and Drake is killed, Vasquez is being held back by Hicks near the door of the APC. He yells that Drake is gone and she says, "No he's not," but her mouth does not move in sync with the line.
While Frost is falling after being set on fire and falling over railing, just before the shot cuts away, in the bottom left corner you can see a crewmen moving and a boot sticking out from the shadows.
When Newt is cocooned and Ripley finds her, two suspension lines are visible on the first alien that comes bounding in off of the walls to attack them.
When the team is descending down to the planet, camera lights and screens are clearly visible in the reflection off Ferro's mirrored sunglasses, and in several shots.
[1:01:00] When Private Frost gets burnt and falls to his death, you can see a camera man's leg.
(at around 26 mins) After Spunkmeyer loads the missile, he calls "Clear behind!" and walks the power loader backward. A thick cable can be seen attached to the loader's left foot. (The loaders are supposed to be free-standing.)
There are several instances in the film of someone shooting an alien at point-blank range, yet not getting any of their acidic blood on them.
In the airlock shaft scene at the end, it is preposterous to think that Ripley could hang on to the ladder as the ship is violently decompressing, whereas the Queen Alien (which is hundreds of times stronger than Ripley is) could not. Not only could Ripley hold on, but she manages to climb back up out of the shaft while the decompression is taking place, making it even more absurd.
When the alien queen follows Ripley up in an elevator, there is no way it could know what button to press, yet it follows Ripley's elevator directly to the same level.
According to the Alien Universe Wikia, Xenopedia, the Queen Alien weighs about ten tons. How she can hide inside the landing gear of a drop ship and no one notice the extra weight or imbalance in the ship's aerodynamics would be impossible. (Note: the ten tons may include a functioning egg sac, which the Queen had left behind for the chase. However, the Queen would still be heavy enough to affect the weight of the ship)
During the initial assault on the atmosphere processor, Bishop and Spunkmeyer are shown talking in the med lab. When Farro and Spunkmeyer board the drop ship, they are going to collect the rest of the team and return to the Sulaco. Bishop should have been on the drop ship with them.
However, Bishop isn't shown on the drop ship, and he has no injuries/damage after the drop ship crashes, He didn't mention being left behind, either.
Did Farro and Spunkmeyer leave him behind, or was he miraculously untouched in the crash? Neither of these makes sense.
However, Bishop isn't shown on the drop ship, and he has no injuries/damage after the drop ship crashes, He didn't mention being left behind, either.
Did Farro and Spunkmeyer leave him behind, or was he miraculously untouched in the crash? Neither of these makes sense.
Before the marines' final stand in Operations, their motion trackers cannot tell if the aliens are on the same level. Therefore the aliens' ability to surprise and slaughter the marines hinges entirely on character error. Ripley references the earlier scene, in which they examined the floor plans, with the possibility that the aliens came in through something they missed or was not in the plans, that being the space above the suspended ceiling vents. By this time, the stress of the situation could have caused everyone to forget this, unlike the calmer floor plan scene.
During the inquest, Van Leuwen refers to Ripley's company ID number as NOC14472 while the data screen in the background displays NOC14672.
Ripley seems to have a remarkably forgiving attitude toward The Company, considering she saw a directive issued by them to Ash in 'Alien' that stated 'crew expendable' with regard to herself and her shipmates.
The crew is deep in black space, landing on a planet obscured by clouds. Ferro normally would not need sunglasses. Obviously this is a fashion statement of hers. However, Ferro is a trained pilot in the marine corps. Making such "fashion statements", especially ones that could impact on her vision and therefore her ability to do her job, would never be tolerated in such a disciplined organization.
When the marines first enter the building, their 'mission time' indicators constantly move back and forward through time by as much as 12 minutes (from 5:21 to 5:33).