When Mikey goes to the animal cage in the classroom, he is wearing a black shirt. When he turns around to see the teacher, his shirt is red. When he comes to talk to the teacher, the shirt is black again.
Mikey shoots an arrow at Jessie and misses hitting the picture of the two pinned to the door. The arrow changes places between shots. In the initial shot, it's through her chest, but in the following shot it's above her head.
The police and detectives don't check Mikey for fingerprints on the murder weapons after he is found in the closet, it it was a real murder crime scene he would have been tested.
Near the end of the film, when Neil parks his car in the garage, the position of Mikey's bicycle changes between shots.
In the opening credits Ashley Laurence's name is spelled Ashley Lawrence with a 'W'. Her name is really spelled Ashley Laurence with a 'U'.
Mikey's hands are wet with blood when he picks up Mr. Jenkins' gun so surely when he replaces the gun, still with bloodied hands, it would be covered in blood but it is apparently clean and Mr. Jenkins notices nothing amiss.