Continuity mistake: After the car chase, the front plate of the taxi is hanging on one side. Moments later, while driving the cab and fighting the other "alien," the plate on the taxi is in a different position (semi-fixed). (00:46:00)
Other mistake: On the desert road with the 3 black SUVs coming from behind, Jack says he will let them pass. As he slows down, a shot of the speedometer is shown, where you can see the gear indicator is in neutral.
Factual error: After the chase scene through the desert and arriving at the abandoned old house, the children hand the taxi driver all of the money they received from the ATM earlier in the movie. When the taxi driver examines the money, it is all hundred dollar bills. No ATM in Las Vegas, Nevada dispenses hundred dollar bills.
Revealing mistake: When they drive out of Vegas, headed for Witch Mountain, there is a shot that appears to be made with a camera on a mobile cherry picker. The camera sweeps across the road and up high, watching as the vehicle passes under the camera. As it passes the camera, you can see a trail of dust that was kicked up by the cherry picker. It crosses the road and is still floating in the air from a recent crossing.
Revealing mistake: When the black guys shoot the four as they are about to enter the spaceship, watch carefully as the bullets hit them in slow motion. Because the camera moves backward slowly, the bullets move backward and then go forward at times. This is probably because the bullet animation is an overlay to the video.