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: Why, in the most secure computer room you could find, with all those detection devices, did they not install video cameras? There is no risk of a security breach as they could be locked into a position that would not show the computer screen or keyboard but would show that an intruder had entered the room.
Stupidity: There's no reason whatsoever for Mr. Walker to trash the other rooms of David's partners. Someone as tough-minded as Mr. Walker would have been able to recognize the consequences of his behavior. He could have simply trashed David's room and let it be. It simply seems like a plot device to justify David's gang attacking the Walker residence.
Stupidity: Carl buries the case with the money in a field by a fence. He looks at the fence line and wonders how he is going to be able to find the spot when he returns. He plants an orange ice scrapper to mark the spot. The scrapper is only 6-8 inches, but it's the middle of winter in Minnesota. The scrapper will be buried by snow. He should have marked the spot by tying something on the fence. (01:18:26)