(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 40 mins) Just before Ripley enters the Narcissus (shuttle), she is carrying Jonesy's carrier with the "window" to the front. The next shot shows her holding the carrier with the window to the rear.
(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.
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 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 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.
The statement; "The ship will detonate in T-10 minutes" is incorrect.
It should simply say "in 10 minutes."
The use of "T-" and a time value just means that an activity is currently that amount of time. e.g. Lift-off is in 5 minutes from now.
One could argue that the turbulence during the landing on the alien planet should not match the ones during take off as they are not triggered by the same mechanism.
(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)
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.
Several shots early in the movie where the crew are at their stations the monitor images are projected onto their faces. This is impossible - they would have to look into projectors instead of monitors, so this effect just doesn't make sense. However, this is a stylistic choice made by the filmmakers to illustrate a futuristic computer display screen, and is therefore not a goof (a similar effect was employed in the film "2001: A Space Odyssey" and later in the TV show "Babylon 5").
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).
The prop fire extinguisher that Parker uses to immobilize Ash, while seemingly quite heavy when first wielded thanks to a post-production metallic clang, proves to be very lightweight later in the battle with no added sound effects: its natural impacts sound neither metallic nor heavy.
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.
The self-destruct instructions at the left are in English and complete. The instructions on the right - which are represented as instructions for canceling self-destruct, but they are actually a repetition of the same instructions in French, with the exception that two lines are repeated and the instructions are incomplete.
Just before the alien kills Brett - there is a close up of the alien with fog from dry ice at the aliens head. This is a reused shot from later in the film when the alien kills Parker. The fog comes from a pipe the alien broke when it attacked Parker. There should be no fog present in the landing leg bay where the alien kills Brett.
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 the Narcissus shuttle jettisons away from the Nostromo, during the first few seconds Ripley looks up at the underside of the ship. The point-of-view of the forward port's frames are the same looking-up as directly jettisoning away. The perspective should have been angled upwards.
(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.
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.
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 20 minutes.
(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.
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."