Revealing mistake: In one scene, two of three prisoners get killed by the alien. The last one is sprayed with blood and tries to escape in panic. With his left arm, he leans upon a half-open, hydraulic-powered metal door. However, it moves a little to the left, and as the prisoner takes his hands off, it immediately snaps back to the right. Now even if you suppose that there was no hydraulic pressure behind it, it should not have moved to the right again. It is very obvious that this door is just a light-weighted part of the scenery.