Factual error: When the crew shoots Kane's body out into space, the door opens up and "explosive decompression" causes Kane's body to fly out into space. At the beginning, the body slowly rotates. Then as it moves further away, the body begins flipping much faster. The reason something would increase speed in rotation like that is due to aerodynamics. An object traveling through space would simply keep the same rotational velocity it begins with since there is no air or other influences (gravity, etc.). The explosive decompression may cause it to increase rotation speed, but by the time it begins to flip, any air would have dissipated into space and not work as a column of air/wind to force the body into a flip.
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)
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.