Other mistake: In the scene where Ripley throws the basketball at Johner, Christie says to Ripley, "Ok, I've got a new game, tag .." He hits her with the handles straight in the face once, he misses the next shot, misses again, and Ripley hits him to the ground, but you neither see or hear the handles fall. (00:23:15)
Other mistake: While fighting Mr. Freeze (and Bane) at the Gotham Observatory, Batman, Robin & Batgirl all have batsuits on, with silver bat (or robin) symbols on their chest. But while they are giving Alfred the cure to Stage 1 of McGregor's Syndrome, for some reason they have changed clothes before giving him the cure. Because now (other than the fact that they have all taken off their masks) Robin has on his original robin suit (with the red robin symbol across his chest) and Batgirl has on a gray t-shirt. (01:55:25 - 01:56:15)
Other mistake: Toward the beginning, when the characters wonder aloud what the rumbling noise is, someone suggests that it's the ventilation system, to which Rennes responds "No vents." However, in the passages between rooms there are panels with ventilation slits in them (which you can plainly see even as Rennes says the line), and it's pretty impossible that the characteristically level-headed and observant Rennes wouldn't have noticed that.
Other mistake: Just after Rico and Ace rescue Carmen, the other guy has hold of the nuke, which explodes. Watch when the rocks collapse behind the three characters, you can see a nice square block lower from the ceiling, almost like a door being lowered. The crew could perhaps have made it look a little more realistic. (01:55:00)
Other mistake: At the end of the movie when the truck falls off the bridge and hits the water, the hood breaks off and you can see that there is no engine or transmission in the truck. (01:28:40)