Character mistake: When Ripley interfaces with Mother for the first time and sees the special order, an instruction reads as "Insure return of organism" rather than "Ensure return."
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)
Character mistake: The menu selection screen on "Mother" has "L. ALLIGNMENT" mis-spelled. (00:08:55)
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)
Character mistake: When the crew come upon the space traveler in the derelict space craft one of them says he is fossilized. For fossilization to occur the tissue must be replaced by minerals from surrounding materials. That is why fossils are found in the ground. When the creature dies it must be covered with dirt or mud before it decomposes. Over time the minerals in the dirt replace the tissue. But the space traveler is sitting in the pilot chair, completely open to the environment and not covered with anything. The term they should have been looking for was 'mummification' or perhaps 'skeletonization'.
Character mistake: Ripley identifies herself as 3rd officer. She is actually a 2nd officer.
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.