Other mistake: When Ripley is trying to deactivate the self-destruct mode, the instructions she follows on the container are actually just the French interpretation of the instructions she read to activate self-destruct before.
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)
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)
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.