Stupidity: In the scene where Billy, Stu and Sidney are in the kitchen (when Sidney finds out they are the real killers), they begin to stab each other as a part of the plan to fool the police into thinking they were almost killed by Sidney's father, whom they were going to frame for the murders. They were planning on killing Sidney and her father, so it makes absolutely no sense that they begin stabbing each other BEFORE killing Sidney and her father, especially since Sidney would be at more of an advantage of outrunning them should she have the opportunity to since they have both been stabbed multiple times.
Stupidity: After hearing the question on the radio station to win the Turbo Man doll and a failed attempt at the pay phone to answer it, Howard and Myron end up running two blocks to the station to answer the question. This is really stupid because the prize was going to the first caller with the answer and in the few min it would take to run to the station two blocks away and finally reach the DJ, many calls would have come in and most likely someone would have won already, way before the men arrived there.
Stupidity: In real life, David beating Gary would be a very big deal since it happened in front of many witnessed and on school property. He would at the very least be arrested or detained sooner or later, if not immediately, for things like assault and possibly reckless endangerment, and could very well have a lawsuit filed against him.
Stupidity: You would think if the security systems in the computer vault have to be off in order for a technician to access the computer then the computer itself wouldn't be accessible if the security systems were on.