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: Dewey and Sidney jumpscare each other at the front door. There's just no possible logical reason for a deputy (or ANYONE) to be holding the mask the way Dewey is in the scene. If he were leaning against the door, he would have lost his balance or reacted in any way to the door shifting. (00:30:30)
Stupidity: When Ghostface stabs Mr. Himbry, the man screams at full lungs; it's 10 minutes to 5 PM and there's a janitor two rooms away, but nobody finds his body until much later in the night - and that is critical to the plan, because he's the designated diversion.