Continuity: Just after the doctor started the post mortem on that little girl, the front of his gown is covered with blood. Yet, a few seconds later, after the two other guys appeared, his gown looks better, with less blood. And during the whole post mortem scene, the blood spot actually changes its size a few times.

Other: The alien that comes out of the dog is a lot bigger than the actual dog just seconds after it appears. This only applies to the theatrical version, as in the extended version the alien comes out of an ox.

Continuity: As Ripley and Dillon lure the Alien into the piston chamber, Morse closes them in just in time. Watch closely - he misses the switch.

Continuity: In the cafeteria scene, Ripley gets a square of what looks like cake and pours half a glass of orange juice. When she sits down her glass is full.

Revealing: 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.