Corrected entry: Ripley's nosebleed: from 1:17:16 (chapter 14: 'A Confrontation with Ash' on the '99 DVD), Ripley's nose starts bleeding, eventually very noticeably. This is almost a full minute before Ash begins attacking her by throwing her twice, which might actually make it bleed. (01:17:15)
Correction: Actually, Ripley's nosebleed was from when they opened the airlock on the Alien and she and Parker were caught in the decompression. That scene obviously was never filmed but the nosebleed was in reference to it. Also, in a cut scene you see Ripley and Lambert talking with Parker over the intercom where he says the Alien is right next to the airlock, apparently somewhat fascinated with a blinking light on in the door.
Correction: This would only be an error if a later scene were intended to show the moment the bleeding is caused by some physical strike, but there's no such moment (and there is the chance that her first shoving match with Ash may have had her head striking his, but it's not a certainty). Still, nosebleeds are commonly triggered by stress in people prone to them. Ripley's nosebleed begins after she learns of the special order (crew expendable) and becomes extremely distraught- and after her physically tossing Ash around (causing his head cut that later drips). She marches off, and in the passageways she can then be seen with the nosebleed.
You're really rationalizing this. Ridley Scott did not make Ridley's nose bleed to show that she's stressed out. It's inexplicable, and was the result of something cut from the film. The presence of it in the film constitutes it as a mistake.
Accidents and unintended effects are not necessarily mistakes. The nosebleed may have been intended as one thing - a reference to another scene - but became something else by that other scene's absence: a detail that helps sell the realism of the moment precisely because it does not feel contrived. Because a random nosebleed that occurs at an inconvenient time whose cause is not immediately obvious is something most people have experienced at one time or another.
Corrected entry: When the Alien appears behind Brett, we first see its tail lowering before the rest of its body, even though the tail is presumably how the creature was being suspended in the air. Immediately after chomping Brett's head, he and the creature are lifted upwards, again presumably by its tail, though it's not long enough to carry them the implied distance and it's unclear what it has latched onto in the first place. This scene starts a tradition of sorts, as both 'Aliens' and 'Alien 3' feature their own scenes in which an Alien grabs a victim and both are subsequently lifted upwards by a great height, presumably by the creature's tail, and the mechanics by which this is possible and even where exactly they're going off to are always obscured to the point where the creature may as well be wearing a jet pack.
Correction: The alien is suspended from the chains above Brett by its feet and hands. In the Director's cut this is more shown. There are also extended versions of the scene which make it more obvious what happens before and after Brett is bitten. The creature uses the chains to escape to the ducts and those are certainly well within reach of the ground. The scene you refer to in Aliens, in which Dietrich is grabbed, shows the alien using its legs to walk backwards up the wall as it carries her, the tail is not involved. These creatures are shown in all the films to possess an ability to grip walls and other surfaces similar to insects, and they are shown with remarkable flexibility, agility, and strength.
Corrected entry: On the special edition: go to the main menu, highlight special features, press right, and you will get the dossiers for the crew.
Correction: This description vaguely resembles an easter egg from the "Alien Legacy" collection or "20th Anniversary Edition" but the wording for just about everything is a bit off.
Continuity mistake: When the ground crew prepares to leave, the captain tells them to take weapons. All three crewman are in suits with side arms on hips. These weapons are never seen the rest of the movie.
Suggested correction: With reference to the side arms mistake the weapons are seen, when Kane goes to look inside the egg before he is attacked he is carrying his side arm and holding it ready.
Corrected entry: The escape shuttle used by Ripley and the cat is buffeted by a shock wave from the Nostromo's explosion, a shock wave that can't exist. In space, there is no medium through which a shock wave can propagate.
Correction: It's not a shockwave. She's just blown up a colossal ship behind her - what she's hit by is the vaporised remains of the ship, rushing outwards from the point of explosion.
You don't see or hear the remains of the refinery hitting the life craft AT ALL so this is an implausible explanation. AND the camera shows the refinery intact as the life craft is pulling away from it...where has the debris come from?
Correction: If sound can travel in space, shock waves can, too. This is standard skiffy film convention. It is not reality.
Correction: It's just an incidental nosebleed that can occur for any number of reasons like dehydration, blood pressure, etc. In addition to the other answers, it seems the film's purpose with the nosebleed is to contrast a sweaty, bleeding Ripley against Ash, who is totally dry aside from a strange drop of white fluid trickling down his face. The juxtaposition is a signal that yes, Ash is indeed "bleeding/ sweating" this white substance as a body fluid and it hasn't just dripped or spilled on him.
TonyPH