Continuity mistake: The fence at the start of the movie changes as Cujo chases the rabbit. In the shot as they run towards it, there is just one straight plank running across the bottom. When they reach it, there is now an askew, diagonal plank attached to the straight one.
Continuity mistake: After shutting his closet door, Tad runs over to his lightswitch. In this shot the lamp on the desk, beside the lightswitch, is standing high up. In the next closeup of Tad reaching the switch, the lamp is in a different position, then switches back.
When the telephone rings, Cujo bounds over and begins barking at it. After he has finished barking, he lingers in the window frame, and then, you can see a crewmember's hand come in, and pull him down from the ledge.