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).
Character mistake: The menu selection screen on "Mother" has "L. ALLIGNMENT" mis-spelled. (00:08:55)
Continuity mistake: When Ash calls Dallas to the infirmary, Dallas appears in a white shirt that has a blue patch or tag above his left/front pocket. Then he and Ripley leave the infirmary and have an argument in a corridor. Suddenly, Dallas' shirt has a gold patch where the blue one was moments earlier.
Revealing mistake: When Parker shoots the blowtorch at Ash, Ash's face begins to melt off. When the skin melts off we see, in slo mo, that the dummy they used has a sealed mouth. (01:27:25)
Revealing mistake: At the start of the film where we see the cockpit for the first time, you can see the camera shake as it bumps into something whilst it pans towards the seats. The sound of the bump can be heard as well.
Continuity mistake: When Ripley is in Mother, you can almost read the words on her face as if a small projector is beaming these words on her. The screen she is looking at has 4 lines of green text. The image on her face is 1 line of white text. (01:19:15)
Audio problem: When the crew are dining together just before Kane meets his grisly end, the scene cuts from a long shot, where Parker is saying something, to a closer shot, with the conversation continuing as before. The problem is that Parker's voice can still be heard, but his lips aren't moving.
Continuity mistake: In the escape pod Ripley presses buttons to release gases onto the alien. Watch her helmet between the cuts. During the first two gas releases the inside of her helmet is foggy and streaked; cut to the alien reacting to the third gas and when the camera returns to Ripley the inside of her helmet is clear.
Continuity mistake: It can be seen multiple times throughout the film towards the end that there is no cat in the box. This is best seen when the front or back of the box is toward the screen when Ripley is moving down a corridor. (01:36:40)
Audio problem: When they are looking at the molecular acid eating through the decks of the ship from the facehugger, Parker says the words "Take a look at this man!", but his mouth is saying something completely different and much shorter.
Visible crew/equipment: As the Alien drops down behind Brett (before it kills him), you can see wires attached to the Alien's back.
Continuity mistake: When the ship warms up before the crew is revived, a computer workplace is shown two times: in the second instance, an empty coffee mug sits before the right side of the screen that wasn't there before.
Other mistake: In the opening bridge sequence when the Nostromo's computer wakes up, text is displayed on the monitor. The scene cuts to a shot of the reflection of the text in a helmet's visor. The reflected text should be reversed, but it is not. (00:02:00)
Continuity mistake: Just after Dallas is taken by the Alien in the air shaft, the crew are round a table discussing what they should do next. Ripley has a visible injury to her right cheek, a round swollen injury. In the next scene the injury has gone. It's possible that it was caused by accident filming the scene when she was attacked by Ash with the rolled up magazine, although that happened later in the film.
Character mistake: While entering the atmosphere of LV-426 when the ships starts to shake in a turbulence, there is a close-up at Ripley's screen showing the terrain and at the bottom of that screen we can see "WEYLAN YUTANI NOSTROMO 180246." The correct name of the Company is WEYLAND YUTANI and Nostromo's registration number is 180924609. (00:16:35)
Continuity mistake: When Ripley sits in the escape pod, she secures her seat belts. As she flushes the alien the belts are on then off then on again.
Continuity mistake: After Ash is decapitated, Ripley says they are going to blow up the ship. In the shot where Parker looks back, Ash's head is slanted to the side. The shot cuts back to Parker where he turns on the flamethrower and incinerates Ash, whose head is suddenly flat on the table. (01:25:40)
Continuity mistake: As soon as Ripley finishes her final conversation with decapitated Ash, she unplugs him and says "We're gonna blow up the ship." A second before she says this, Ash's eyes are black and completely different to what colour they were before. (01:25:15)
Continuity mistake: When Ripley is trying to re-activate Ash, Ripley flattens his head onto the ground. Her left hand is placed on Ash's head and her right hand is placed on the skin below Ash's neck. As soon as Ripley gets the head fixed on the ground, she begins to remove her right hand from the skin below Ash's neck. In the instant shot following, her hand is suddenly back on the skin below Ash's neck. (01:23:05)
Other mistake: When Dallas is in the air vent, his and the alien's location are both being tracked with the movement sensor held by Lambert. Whenever he crawls, his movements match up with the sensor. As he climbs down the ladder, his movement on the sensor goes downwards as it keeps in sync with Dallas himself and also shows that anything sensed on the sensor would be lower down on the sensor's monitor if someone or something was located on another level of the vent. The shot after Dallas says "Am I clear Lambert? I want to get the hell out of here" shows the monitor on the sensor and we can see the alien approaching Dallas on the monitor, with both Dallas and the alien on the same level on the air vent. Dallas then climbs down another level, and it is that the alien was in fact a level below Dallas. The problem with this is, if the alien was approaching Dallas from below, then the monitor should have showed the alien to be a few spaces below where it was on the monitor and not the same level as Dallas. (01:12:55 - 01:13:55)
Answer: According to the commentary on the DVD, Ridley didn't add this scene to the original cinematic release because he thought it revealed the true horror of the Alien too soon in the film. The scene is quite early in the film and he thought revealing the fully matured Alien at that time would reduce the viewer's fear.
I had watched Alien several times before I noticed the Alien hanging there.At this point the Audience have no idea what the Alien looks like, they're looking at pieces of science fiction equipment put in by the production crew that they can't relate to, so for all they know the Alien could just be a piece of kit hanging there.