The flag in the biology classroom disappears and reappears several times.
In Callie's nightmare, she jumps into a Ford Fairmont to escape the killer. However, when she tries to start the car, the steering wheel clearly has a Pontiac emblem.
A trapdoor in the bottom of the sandbox large enough for a man to go through would quickly allow most of the sand to spill out, yet the sandbox remains full.
After Mr Birnbaum wakes Porter up with the chemical, the boom can be seen, as well as when the teacher puts his arm around Susie.
Early on, we meet Oliver, an actor playing Steve the policeman, who investigated the 1982 murders when they happened. Later, though, it is revealed that Steve and Cathy were high school sweethearts at the time of the murders. If Steve was still a high school student, he would not have been a policeman yet, and could not have investigated the murders.