Goofs

When the police ring the doorbell, the Christmas lights are flashing, but this was after Mr. Beindorf had shut off the power to the house.

Stacey's nail gun would require air pressure to make it work. There is no air hose hooked to it, making it impossible to use.

When Mr. Krupp gets through the concrete, it appears that he'd been digging straight through, but when Mr. Finley goes through and gets stuck in the concrete, he's coming up through a hole made in the concrete-laid ground level, rather than perpendicular to the ground. In addition to it being a factual goof, given the previous scene, it would also be physically impossible for him to climb through; his back would have to be bent at a 90-degree angle at one point to accomplish this feat.

There would be no cobwebs, and certainly no live rats, in a laundry chute that had been completely sealed years earlier as shown.

The string pulling the chair away from the door can be clearly seen, as Mr. Beindorf is trying to break down the basement door.

When Grover and Stacey are riding their bikes into the driveway of their house at the beginning of the movie, a very tall mountain can be plainly seen in the background to the right of the house. There are no mountains in or near Defiance, Ohio.

When Matt and Grover are sitting down to eat lunch in the school cafeteria at the beginning of the movie, Matt swings his head around and looks straight at the camera for a moment, sort of smiling.