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)
Continuity mistake: While Parker is decapitating Ash with the fire extinguisher his T-shirt is not torn, just after he says "Ash is a goddamn robot" his T-shirt is now torn and held together with a safety pin. (01:23:20)
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 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)
Continuity mistake: When Ripley reconnects the dismembered Ash droid, the switch from dummy to actor is blatantly visible. A few minutes later the android is incinerated, at which point the dummy head has reappeared and the android's left arm has disappeared. (01:24:39)
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: 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)
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: 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)
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: 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)
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)
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)
Continuity mistake: In regards to the self destruct mechanisim of the Nostromo, It is said that the time from the setting to detonation will be 10 minutes. This is also confirmed by the voice of "Mother" who says, right after Ripley initiates the sequence, that the detonation will take place in "T-Minus 10 minutes" However, in the next shot you see one of the monitors which shows a countdown with 10 minutes and an extra 30+ seconds on it. (01:35:55)
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)
Visible crew/equipment: When Ripley is heading towards the Shuttle towards the end, just before she finds Jonesy, there is a POV shot where the camera is walking towards a ladder. If you look at the floor you can see the dolly that the camera moves on. (01:36:05)
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)
Continuity mistake: When Ripley runs back into the control room to abort the self-destruct, one shot shows the timer reading 5 minutes. The shot then cuts to Ripley pulling a lever back to where it originally was and yells out "Mother, I've turned the cooling unit back on! Mother!" The shot then cuts back to the timer where it can be seen it has only gone down by 2-3 seconds, but all of what just happened took longer than that. (01:38:40)
Revealing mistake: In a scene near the end, the camera operator appears to bump into Sigourney Weaver from behind, prompting her to momentarily break character. Weaver had been frantically running down a corridor with the camera following closely behind. When she reaches a ladder, the view passes her for a split-second and bounces back a step from an apparent soft collision. Weaver glances back at the camera with a look as if meaning to say, "Are you all right?" but immediately resumes climbing the ladder. (01:39:35)
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)
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.