Plot hole: When Ripley goes to access the computer Mother, the entry door opens and closes with a hissing noise, a couple of minutes later you see Ash next to Ripley with a smile on his face. How did Ash manage to open the same door without it making the hissing sound? Ash could not have been in the room before Ripley came in - once Ripley has sat down at the console, it rotates on a 90 degree angle, this would have made it impossible for Ash to be hiding behind a desk or a computer console without being seen by Ripley. There are many doors on the ship that make the same hissing sound when the doors open, an obvious example is the infirmary door. And Ash couldn't have disabled the pneumatics of the door, because when Ripley exits the room, the hissing noise is heard again. (01:18:40 - 01:20:00)
Suggested correction: There's no evidence that sound happens every time the door opens. It may be a pressure release sound, but not enough pressure has built up since Ripley opened the door to make the sound. It's like flushing a toilet twice in quick succession, before the tank has had a chance to refill. The action is the same, but the same mechanism is engaged; it makes a different sound.
Plot hole: The shuttle "won't take four" crew members. Considering it only has two cryotubes (and hypersleep appears to be necessary for any hope of survival), it doesn't seem to accommodate three people, either. (Admittedly I would still go for it, too, if I were up against a 7-foot-tall acid-blooded alien monster with two sets of jaws, but there doesn't seem to be much difference whether there's three or four people onboard).
Suggested correction: As was standard, the Narcissus (which was the name of the shuttle) was designed to carry a crew of three, although the interior could be easily modified to seat more crew members if necessary. For long-duration travel, the shuttle contained two stasis capsules, although long-range interstellar transit was not the vessel's primary purpose. So in a life-or-death situation, four people intending to use it to get off the Nostromo isn't a plot hole.
I'm not sure I understand the correction. The reason Ripley rules out escaping on the shuttle at first is because it does not accommodate four crew members. When she later changes her mind after they've destroyed Ash, the natural assumption is that the shuttle has become a viable option now that they're down to three, but we later see that wasn't actually the case, because it only has two cryotubes. A better argument, I think, is that at that point they simply no longer cared.
