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.
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Mistakes
As Ripley and Dillon lure the Alien into the piston chamber, Morse closes them in just in time. Watch closely - he misses the switch. See more...
Alien 3 mistakes
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: 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.