Continuity mistake: When Jack is having a seizure at his job he stuff some aspirin in his mouth. When the manager says "He is spaced out." it cuts to the girl saying "I'll handle this.". At that point Jack has got two aspirin hanging out of his mouth. It cuts and the aspirins are gone.
Continuity mistake: Just after the scene where Jack Putter gets tazed by Lydia Maxwell in the restaurant, Mr. Igoe initially hauls the unconscious Putter over his left shoulder. In the following shot you'll notice that when he exits through the back door, he's carrying Putter over the right shoulder. This mistake was pointed out by the director (Joe Dante) himself on the commentary track of the DVD version.
Continuity mistake: When Pete and Dr. Niles are in the car discussing the project, the background keeps changing depending on the camera angle. They're suppose to be stuck in traffic, driving near a giant hillside. But in some angles there's no cars next to them and it's wide open space with no sign of the hill. And the cars around them change.