(at around 52 mins) During the conversation about Brett being a parrot, the length of his cigarette constantly changes.
(at around 2 mins) During the introduction scenes of the Nostromo, the stars in the background are maintained even though the spaceship is shown from different angles.
(at around 1h 45 mins) Near the end when Ripley is in the shuttle closet putting on the space suit there are 3 axes hanging on the wall. The number of axes goes down to 2 while she suits up, and the axes have completely vanished when she puts the helmet on.
(at around 20 mins) When the members of the Nostromo are exiting the ship to leave to explore the Alien ship,the wind is blowing at high speeds, yet when the steam is shown from the lift it is just drifting down rather than being blown away at high speed. other scenes show the high wind blowing.
(at around 1h 19 mins) When Ripley consults Mother, she puts her left hand on a ledge and begins to type with her right. In the close-up she is typing with both hands, but when we cut back to the wider shot, she is still leaning on the ledge with her left.
There are various spelling mistakes on the computer screens in the Nostromo:
- (at around 9 mins) When Dallas is going through the computer login sequence to communicate with Mother, and all the green-lettered commands are flashing by, the word "ALIGNMENT" is spelled "ALLIGNMENT".
- (at around 1h 19 mins) When Ripley is accessing the company's secret orders, the screen says "OVERIDE" instead of an "OVERRIDE". In the same scene it then says "PRIORITY ONE - INSURE RETURN OF ORGANISM". "Insure" is the wrong verb (suggesting that the company should take out a policy with an insurance company). The statement should be, "ENSURE return of organism."
(at around 3 mins) A moving dippy bird toy is shown in the opening scene. Dippy birds require a fluid that evaporates in order to work, creating a temperature differential between the bird's head and tail. Given the length of their hibernation, the fluid should have long ago evaporated, and the bird stopped moving.
(at around 1h 50 mins) Because she's no longer in a flight seat or in any way strapped in, when Ripley fires the shuttle's engines, the g-thrust should throw her against the aft bulkhead, but it doesn't.
(at around 16 mins) One could argue that the turbulence during the landing on the alien planet should not match the ones during take off (at around 51 mins) as they are not triggered by the same mechanism.
In the takeoff sequence the "entering orbit" announcement is heard just after emerging from the clouds - this is impossible because the atmosphere ends at a much higher altitude than the top of clouds/dust layers, and the orbital speed cannot be reached in air even above a small planetoid, moreover, there must be a horizontal acceleration stage to gain the speed necessary for the orbital insertion.
(at around 11 mins) Dallas informs the crew that Mother, the ship's computer, has interrupted their journey having intercepted a transmission of unknown origin, an acoustical beacon that repeats at intervals of 12 seconds. Acoustics is a branch of science dealing with sound. Sound is a mechanical wave that requires a medium, like air or water, in order to propagate. There is none of that in space. However, it is obvious Dallas was referring to the signal being one that was audio only rather than one containing video.
(at around 33 mins) A close-up of the Alien egg when Kane is looking at it shows water droplets falling upwards off the egg, revealing that the shot was done with the camera upside down. (according to Trivia, this is an intended effect by Ridley Scott)
When Ripley, Lambert and Parker decide to abandon the Nostromo, the Alien kills Lambert and Parker when they go to get coolant for the shuttle's air support system, due to the fact that there were only two hypersleep chambers and they would all be awake for an indefinite period. Ripley leaves without the coolant, which was no longer needed as she was alone and would be going into hypersleep by herself.
(at around 20 mins) Before embarking on their walk to the Alien ship, Captain Dallas says that they should 'break out the weapons'. Those weapons are never seen or spoken of again after that. They end up having to make crude weapons (cattle prods) to confront the Alien when in actuality the ship had a small cache of arms. However, the whole point in avoiding those arms is the danger posed by the Alien's acidic blood. Their plan, instead, was to drive it into the airlock and flush it off the ship.
(at around 20 mins) The headlamps on the actors EVA helmets, shining down on their faces, is effective for dramatic images on film, but in reality the glare would make it impossible to see beyond their visors, especially into the murk of the planets atmosphere. However this is a stylistic choice made by the filmmakers and not a goof.
(at around 1h 35 mins) When trying to abort the countdown, the cancellation instructions Ripley follows with her finger are actually just a French translation of the detonation instructions she followed earlier.
(at around 10 mins) When Parker and Brett are walking through a long hallway and complaining that none of the other crew "come down here", their reflection can briefly be seen in a mirror behind them. Mirrors were used to make the set hallways look much longer than they actually were.
(at around 39 mins) The two holes caused by the facehugger's acid blood in the ceiling of the two different decks have exactly the same shape (re-used shot).
(at around 1h 45 mins) Despite the humid temperatures of the ship during the detonation sequence, and the fact that Ripley is seen coated in sweat, including her nails, when she disrobes in the escape shuttle, there is no perspiration anywhere on her under garments, not even the armpits.
One of the moons in the system containing LV-426 (the target moon) is out of phase with the others.
(at around 1h 35 mins) The self-destruct procedures on the housing lid are printed on a backlit black and clear sheet of acetate. The coloring of the letters was applied by hand using colored felt markers. The scribbling lines and overlapping ink is clearly visible.
When Ripley reads Order 937, it says insure when it means ensure.
(at around 37 mins) When the rest of the crew is watching Dallas and Ash try to get the facehugger off of Kane, Parker asks, "How come they don't freeze him?" but his lips don't move when we hear this.
(at around 55 mins) When the crew is sharing a meal after Kane regains consciousness, Parker is shown to be talking. The shot changes to show the crew from a closer angle, and although we can still hear Parker speaking, he is shown to be laughing softly and not speaking at all.
(at around 20 mins) Heavy winds are heard when the rescue team steps out of the ship but the straps in the airlock do not move (the wind should have engulfed in it when the doors opened).
(at around 46 mins) When the Facehugger has disappeared from Kane's face, the audio for his breathing sounds like he is breathing through his mouth but his mouth is closed.
(at around 1h 35 mins) A crewman with a black panel of some kind is visible through the smoke as Ripley runs through the corridors at the end of the film.
When Dallas is crawling around through the bowels of the Nostromo before he is taken by the alien, you can see a dolly track lining the floor.
(at around 57 mins) Just after Kane's death scene, but before his 'burial' in space, the crew briefly talk to each other asking if they see the alien. As they do this, the camera moves about and rotates in a corridor intersection. Just as this shot ends, some equipment can be seen on the left edge bobbing about. This may even be a mic of some kind.
(at around 14 mins) During the landing sequence, Kane issues the instruction "Roll 92 degrees port yaw." Roll and yaw are two separate directional axes. The correct instruction (and what the ship actually does) is "Roll 90 degrees port."