Revealing mistake: When Hillard throws the basketball at Ripley, you can clearly see the white tape on Ripley's hand which was used to catch the ball. (00:23:10)
Revealing mistake: In the underwater scene, Johner fires a grenade launcher at two of the aliens that are following him. Watch closely and you'll see the grenade pass through one of the aliens' head for a few frames without damaging it before exploding. Slow-mo might be helpful to spot it but isn't vital. It's perfectly visible at normal speed.
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)