Continuity mistake: When the newborn alien is getting sucked out of the ship, the women have their hair pulled and strained in the wind. Yet, when we see a video of the women, their hair is not moving at all.
Factual error: In reality, the normal air pressure of a room (about 14.7 psi) would be nowhere near enough to force the newborn's body out through such a tiny hole.
Revealing mistake: During the brief firefight in the mess hall, Distephano is disarmed and drops to the floor. After the firefight, he surrenders and stands up with his hands behind his head. The same shot of him standing up is reused in the security footage Gediman watches from the control room seconds later. (00:39:05)
Suggested correction: There could be a delay of several seconds in the footage, its probably a really outdated security system.
Other mistake: When Ripley pulls the knife out of her hand the blade is smoking as her acid blood is melting the steel, however shortly after she touches the girls face and hands, and the acid blood has no effect.
Other mistake: The remaining crew of the Betty, plus 2 others, have to swim 90 feet underwater. This they do, but once they are out of the water, the only one who is wet is Winona Ryder.
Character mistake: When Ripley asked the doctor how they "got" her, he replies from blood samples on Fury 16. This is in reference to the planet in Alien 3, but it was called Fury 161. (00:11:00)
Continuity mistake: After Gen. Perez pushes the stacks of money towards Elgyn, they are right on the metallic edge of the table when the general offers a drink to Elgyn. In all of the following shots, the stacks have moved and are further in on the table, not on the metal anymore.
Visible crew/equipment: After Ron Perlman has uttered the line "f***ing waste of ammo" in the clone room there is a shot that starts at ground level and rises up. You can see a ray of light heading towards the left of the picture being obliterated by the camera's shadow as it rises.
Audio problem: In the nest, the creepy guy talking does not sync his words with his lips half the time.
Visible crew/equipment: When the android is connecting to the ship, in a lot of close ups what looks like a stage lamp is reflected in her eyes. There wasn't anything in the room that looked like that.
Continuity mistake: Ripley puts the knife in Winona Ryder's hand through her own palm. Initially there is blood on the knife and on Ripley's hand, plus a hole where the knife went through, but all three of these come and go during the next few minutes.
Continuity mistake: The scene when the surgery is performed to remove the baby alien, in the first shot shows the laser cutting below Ripley's breast area. In the next shot you can see the shape of her left breast under the surgical protective material.
