Continuity mistake: When Ash has gone crazy and is attacking Ripley, his white android juice/sweat/fluid is flowing from his forehead. After he throws Ripley into the room with the nude pictures on the wall, there is a new shot. In it, Ash is walking into the room, but there is no android fluid on his forehead. (01:21:35)
Continuity mistake: In the air duct when Dallas touches the slime, the amount of slime on his hand changes between shots.
Visible crew/equipment: When Kane is struggling on the table shortly before the alien rips through his chest, just before the first patch of blood appears, his T-shirt rides up and something strapped to his chest can be seen. This is some sort of pack containing fake blood that discharges onto his T-shirt. (Best viewed in slow motion.) (00:55:40)
Continuity mistake: When Lambert deactivates Ash by using a cattleprod, you will notice that she is wearing brown cowboy boots. Minutes later as the Alien goes to wrap its tail round the back of her legs, she is now wearing a sneaker - though you only see the toe cap it is clearly not a brown cowboy boot. (01:23:55 - 01:33:40)
Visible crew/equipment: Following the chest-burster scene, as the camera pans around the darkened corridors on the Nostromo just prior to Dallas ejecting Kane's body, what appears to be a boom mic is visible only for approximately two seconds to the left of the screen. (00:57:10)
Revealing mistake: They use the same shot three times when showing the acid eating throughout the ship. After they try to cut the finger off the face sucker, acid begins to burnt through the decks. When they have gone down 1 deck and look up they see where the acid came through. They then move down 1 more deck and look up again. The acid is just beginning to burn through. Then they look up again and it is the same shot that was used before. The same hanging acid on the right with the drip on the left. After the acid has neutralized, they look up at the hole and it is the same shot again. (00:39:50)
Visible crew/equipment: When Ripley is in the escape pod and has the space suit on, she hits the button to open the door and blow the alien into space. In the shot just as the alien is getting sucked backwards, you can see the wire coming out of its back. (01:51:30)
Revealing mistake: As Ripley stumbles away from the self-destruct system, the cooling rods are not visible. A few seconds later, in close-up, they can be seen to be above the edge of the compartment they are in. (01:32:10)
Audio problem: When Tom Skerritt first "interfaces" with Mother after being awoken, he says, "Good morning, Mother" - it is not even close to being Tom Skerritt's voice. According to the book 'The Making of Alien' this line was dubbed in by Ridley Scott as Tom Skerritt wasn't available at the time.
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)
Factual error: When Ripley is trying to set the detonation of the Nostromo, the sequence begins before she activates the fourth and final fuse on the self-destruct control panel. (01:35:50)
Revealing mistake: When Ash attacks Ripley and Parker beats him off, the android gets up again and tackles Parker. Look closely as Parker crawls up against the wall...the body of Ash isn't a body at all...it's half a mannequin that doesn't even have a lower body connected to it, and it is visible when Parker's legs spread and you can see that there is nothing under Ash's torso except a big hole where the lower half should connect. (01:23:45)
Continuity mistake: When Dallas, Kane and Lambert are exploring the alien spacecraft, you can see that there are wipe marks inside of their helmets that change between shots, most notably on Kane. This is because the helmets were built with out sufficient exhaust systems for the actors. Of course, the characters wouldn't be able to wipe the inside of their helmets with them on.
Character mistake: When Ripley is talking to "mother" by typing, it types "Emergency command overide 100375". "Override" should be spelt with two R's. (01:19:34)
Other mistake: The voice of Mother saying the self-destruct sequence has been activated is heard before Ripley pushes the red button of the last of the four mechanisms used for that purpose. (01:35:58)
Revealing mistake: Parker had hair. When the alien bit through his head the melon they used did not have hair. Also, the seeds can be seen inside the melon when paused at the right moment. (01:33:35)
Continuity mistake: After the alien leaves Kane's face and the rest of the crew are looking for it in the infirmary, there is one shot that bears re-watching. When Ripley is looking upwards over the prone Kane, you begin to see the dangling tentacles of the alien in the background. At the same time, the collar on her outfit begins to pull downward, un-naturally, until the alien drops onto her shoulder. I believe there was some sort of string or rope fed from below, through her clothing, and was attached to the alien and was pulled to have the creature drop onto her. (00:48:33)
Continuity mistake: When Ripley fails to override the self-destruct system she hits the computer with her flamethrower. As this happens it cuts to the screen which displays 10 minutes left until detonation, when really it's under 5. (01:35:15)
Continuity mistake: Before Lambert is killed, the alien twists its tendrils around her leg, and we see that she has sneakers on. However, when Ripley notices Lambert's dead body (hanging?) she is barefoot. [The rest of the scene was cut. According to the novelization, Lambert was actually intended to be shown completely naked and sexually ravaged. Director Ridley Scott had toyed with the idea of creating the illusion of the Alien having raped her before her death. He thought better of it and cut the scene. Still a mistake, but there's why.] (01:33:40 - 01:34:25)
Factual error: In the opening scene we see a bobbing bird on the table. These birds require a wet beak to work thus the glass of water. We find out that the crew is half way home when they are awakened. Later we find that they are 10 months from home, so they must have been travelling for 10 months already, meaning the bird has been bobbing for all that time and the water has somehow not evaporated.
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.