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: 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.
Suggested correction: This isn't a stupidity or a mistake. What else did you expect them to do? Plus, the DJ even later asks if they expected him to actually have a Turbo Doll in the studio, and they both said "yes."
Not only that but, Myron had ripped the phone receiver out so Howard couldn't give the answer to the DJ. Howard and Myron running to the radio station was the only other option they had. At that point, it would only be a matter of if they got to the station in time before somebody could call in with the right answer.
It was a very easy question naming santa's reindeer. Although some people may not quite know them all... most people do. Now if it was a relatively hard question, our two guys probably had a better chance. But all good points made here in all these comments.
Stupidity: This film portrays Michael Jordan and his family living in a relatively modest house in the suburbs. Jordan was one of the most famous people on the entire planet around the time the film takes place. Living in such an area would lead to a never-ending onslaught of overzealous fans, paparazzi, and even detractors.
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)