Revealing mistake: When Mandy falls inside the Porsche, her stunt double is noticeable because she is way older than her.
Continuity mistake: 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. (00:27:00 - 00:27:50)
Visible crew/equipment: When Charlie is in the ventilation system and says, "I could use some help here," the boom mike briefly pops into the picture. (00:44:40)
Continuity mistake: When Veronica arrives at the Hoover Dam and pulls over there are just three people next to the car. A shot later there are 5 people on the left who have appeared out of nowhere.