During the scene when Officer Delinko is chasing Roy in his police cruiser, he backs it into a tree and the bumper of his car is clearly banged in. Then it changes view and he hits another tree with the front of his car. However, when the rear bumper is visible again, there is no damage done to the back or the front bumpers and the car is in perfect shape, as though no tree-hitting had occurred at all.
In the scene where Officer Delinko comes to tell Roy's parents that Roy has been bitten by a dog and taken to the hospital, the officer has a toothpick in his mouth. During the course of the conversation after that, when he has his back to the camera, you do see the toothpick; in all of the front shots of him, the toothpick is missing.
In the scene where Officer Delinko tells Roy's parents that he has been bitten by a dog, as Roy's parents drive up, there is no license plate on the front of the car. However, when Delinko has his back to the camera and the front of the car is in frame, there is a blacked-out license plate on the front of the car.
The goof item below may give away important plot points.
In the scene where Roy, Beatrice and Mullet Fingers are standing in front of the bulldozer, Mullet Fingers is shown barefoot. A few minutes later, just after Chuck Markle is taken away by police, Officer Delinko approaches the kids, and as Mullet Fingers slips away, a shoe is clearly visible on his right foot.