Stupidity: When Cathy, Chris and Carrie go into their late grandfather's room, they discover a copy of the grandfather's will that states that if it was ever discovered that Corrine had children, she would lose everything. Had they taken the will to their mom's wedding and showed it to her fiance, they would have been able to expose the scam much quicker. Instead, they take the dead mouse Fred with them and show it to everybody. What good would a dead mouse do when a will has more leverage?
Stupidity: At the prison yard, when Weiss overhears that the sonic deadline was being deactivated to let new prisoners in, he manages to look at the deactivation code. The guard sees Weiss looking at the code and switches the computer off. If he had changed the code and informed another guard, none of the prisoners would have escaped.
Stupidity: After the Slick devours Laverne, Randy jumps into the water and swims for shore with the Slick following. Even though Randy made it to shore, he stops and turns to confront the Slick, screaming that he beat it. When Randy turns around, the Slick immediately envelopes him. If Randy had just got up and started running instead of facing the Slick, he would have lived.
Stupidity: After Caleb escapes the vampire "family" with his father and sister, he undergoes a blood transfusion which cures his vampirism. As soon as this was done Caleb, his father and sister should have fled somewhere else immediately. The vampire "family" knew where he lived as they kidnapped him in front of his home. Instead, they stick around the home, with no obvious intention of leaving. Of course, the vampires track them down and kidnap Sarah.
Stupidity: When questioning the heavyset witness after his having seen Costner in the hallway, there is no mention of the most obvious identifiable feature...the pearly white Navy uniform he was wearing. Instead, Costner is described as being a common "everyman."
Stupidity: There is no logical reason for a bookkeeper to be present at the liquor deal on the Canadian border and even less of a reason for him to bring the primary ledger with him. This is compounded by the fact this is merely a secondary bookkeeper and the main bookkeeper is subsequently subpoenaed by the district attorney.
Stupidity: When making apple sauce, just slapping a paper lid on the jar (instead of canning it properly) , would soon result in the jars turning to mold.