Revealing mistake: Mirrors were used to make the corridors appear longer, early in the film when Brett is complaining to Parker about the way they are treated by the company you can see their reflections in the mirror behind them as they walk down the corridor. It's only for a second as they are moving off camera but you can see them in their white overalls.
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)
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)
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: 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)
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)
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.
Revealing mistake: The amber warning lights on the ceiling start flashing before Ripley has activated the self-destruct system.
Revealing mistake: As Ripley, Lambert and Parker leave the room after decapitating Ash, Parker turns around and incinerates Ash's body. As soon as the shot starts, if you look carefully just before the flame actually touches Ash, you can see Ash's head (mostly the skin below his neck) has already melted before the flame, or even the heat for that matter, touches him. The flamethrower had only been set off a split second before the fire reached Ash and by that point the head has already began melting, so the heat made from the fire could not have melted his head that quickly.
Revealing mistake: When Dallas is in the air shaft he shoots downwards with his flamethrower, you see the flames that he is shooting from the bottom of the ladder, but you can see a flamethrower shooting upwards from the side of the shaft and it also moves in an arc.
Revealing mistake: You can see the blood was initially thrown on John Hurt instead of coming from underneath his shirt. You can also see the underlying prop wrap that was used for the blood exploding out of him just before you see the alien pop out. (00:56:56)
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.