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: After Dan confesses his affair to his wife, he never thinks about bringing the tape Alex had sent him to the police. This would have been proof beyond a reasonable doubt of her mental instability and harassment of him, and she would have been immediately sought out and likely committed to an institution, and both of the film's endings wouldn't had to happen.
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.
Suggested correction: We never see them put the will away. It's possible one of them had it in their pocket during the wedding scene, but they just never showed it because A. Showing the will would have given motive for their mother to want to kill them, but not have proved that these children were hers, and B. The dead mouse is a more visceral, hate-fueled move that matches Kristy's anger.