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: 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: 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: It's another race of space alien that is also subject to infection by the Aliens. This particular one was infected and moved as far as it could from its race's known space and broadcast a warning before it died. The presence of eggs in the hold may indicate that it was a research ship. The race was never used in other Alien movies because it adds a new dynamic to the plotlines: two alien species, locked in mortal combat and neither particularly friendly with humans. In the Alien pseudo-prequel Prometheus we learn these beings are known as the Engineers and have interesting ties with both the aliens and humans as well.
Phoenix