Continuity mistake: Towards the end when Chris and Gladys escape the giant spider in the mine on a motorbike, Gladys picks up a helmet and says to Chris, "I need this more than you". The helmet she is holding has a cobweb on the cage at the front. But once she gets on the bike with Chris, the cobweb has disappeared.
Audio problem: In the scene where all the towns residents take refuge in the Prosperity Mall, the mall's janitor/caretaker guy says something to the female sheriff with the camera facing him. In mid sentence the camera angle changes to behind the janitor but at an angle - you can hear him keep on talking but you can see that his jaw is perfectly still meaning that he can't possibly be talking.
Continuity mistake: Something weird happens as the sheriff rolls under the closing gate. She's holding a shotgun before she drops to the floor, rolls under with empty hands (seen from both sides), and as she stands up she's got it back. What's weird is that her dropping and rolling is one continuous shot, so she must have deliberately thrown the gun offscreen as she fell to help her fit through the narrow gap. She doesn't throw it through the gap - we'd be able to see it on the other side.
Continuity mistake: When the cop's beeping his horn outside the car as the spiders jump towards him, he's glaring at the house until the shot of the spider landing on his roof, when he's staring fixedly ahead (not at a spider - there aren't any in front of him). Next shot he's glaring at the house again.
Continuity mistake: Just after the female sheriff's daughter is attacked by one of the giant spiders there is a scene where the sheriff is holding a double barrel shotgun and hands David Arquette a pump action shotgun. A few seconds later the sheriff has the pump action shotgun and David Arquette has the double barrel shotgun.