Who, What, Where, Wendigo? - S2-E10
Stupidity: Dwight, Audrey, and Nathan all know Dwight's trouble is that all bullets are attracted to him, so why is he with them when they go looking for the killer, knowing every one is using a gun?
Stupidity: The villain should've removed the tape hidden in the mask once he had it as it was incriminating evidence.
Backlash of the Hunter - S1-E1
Stupidity: Rockford knows almost nothing about the guy who is tailing him except that he noticed him while he was grabbing a bite with his client. Regardless of that, he sends the same client to seduce him, just assuming that he won't be able to recognize her. And it works. Is Lindsay Wagner this hard to forget, that this guy just an hour or two later isn't able to tell it's the same woman his target was meeting?
The Alchemy Murder Case File 4 - S1-E9
Stupidity: Everyone mentions in the first episode that the various doors are made out of different metals (some rather unlikely; a door made out of pure manganese?) but they don't show it, all have the same color and weight (the door made out of lead and the one made of tin would be significantly different, and one is the treasure, 966 kg). More importantly, since it's a known fact that the doors are made out of different metals, each has a number that matches the periodic table, and they are looking for gold, the supposedly hard enigma concocted by the genius alchemist is something a high schooler would figure out in minutes; Miyuki even knows its atomic number by memory when randomly asked.
Episode #1.2 - S1-E2
Stupidity: The part when the Colonel and the governess pose as journalists does not make sense. It's far-fetched enough that after a lengthy interview about the culprit's fortune the topic of his family never came up, considering it was one of the main goals and an ideal question to lead with, but considering that "mr. Todo" does not want a picture taken and has a tight security, there's no way that a photographer carrying a large 1930 camera would have been allowed into the interview room to begin with.
A Deadly State of Mind - S4-E6
Stupidity: In the final scene: Columbo lays a trap for the murderer (George Hamilton). The 'eyewitness' when Hamilton fled the murder scene was a blind man. Hamilton is aware of this. Columbo's gambit depends on Hamilton's assumption that Columbo has coached the witness to act as though he was sighted so as to convince Hamilton that there is evidence against him. In fact, Columbo has the witness' brother, who is sighted, pretend to be the eyewitness and identify Hamilton as the man he saw driving away. When Hamilton falls for Columbo's trap and, believing he is in the presence of the blind man, tries and fails to 'prove' the man can't see. By doing so he reveals his knowledge that the witness was blind, incriminating himself. The problem is, that only a minute or so previously, Columbo had placed Hamilton under arrest, and failed to read him his Miranda Rights. Considering that Hamilton's slip up is the ONLY evidence Columbo has against Hamilton, this was a spectacularly stupid move on Columbo's part. Regardless of how many officers were present to witness this, the fact is that Hamilton was under arrest and had not been advised of his right against self-incrimination; meaning nothing that he said when under arrest can be used against him.
Stupidity: Hard to swallow that a group of vastly experienced policemen smart enough to figure other tricks out (such as the abnormal position of the gun) would be in any doubt about the victim being left or right handed when she is still wearing their watch on the right wrist - and attention is called by Poirot upon that particular detail right from the start.
Camp Redwood - S9-E1
Stupidity: Brooke leaves her apartment window open as she sleeps, even after telling her friends that serial killers (like The Night Stalker) strike during the summer when everybody leaves their windows open.
Day 4: 12:00 A.M.-1:00 A.M. - S4-E18
Stupidity: After leaving the power plant to pursue the terrorists and retrieve the nuclear football, Jack orders the helicopter to take out the first Jeep. After he realises parts of the football are missing, why does he not order the helicopter to pursue and take out the other Jeep, preventing any further trouble from Marwan? (End of season 4's story, most likely).
Stupidity: The brothers are angry at Leo because they say he's responsible for them dying because he chose to save other soldiers instead of them, Leo also blames himself for the same reason and says he should have stayed and gotten them out. But in the flashback it shows Leo helping them and then running off to help other soldiers, after he runs off the brothers are killed just a few seconds later. Even if Leo had chosen to stay with the brother he couldn't have gotten them out in time, the only difference would have been Leo would have died with them, but Leo dies in the same war so he didn't live much longer. Leo's guilt and the brothers' blame don't make any sense at all.
Is Lupin Burning... ?! - S1-E1
Stupidity: Despite the fact that there are 100 (literally) cameras around the racing circuit and (again literally) everyone from the staff to the fellow racers works for the Scorpion gang, Jigen and Lupin's switch can happen without anyone noticing, even if it involved racing side by side for a lengthy period of time.
Stupidity: The inspector knows that his house has been bugged; he does not know how many bugs and cameras are there, he just knows there are some. He does find a bunch of them with great ease when he gets back home - making you wonder how could he have ignored them to begin with, since all it takes is for him to raise his eyes, they are fairly big black cameras who do not camouflage at all for the most part. He then proceeds to make a phone call to the big baddie from his landline. Nobody in their right mind would do such a thing. Heck, you wouldn't want to use your home phone for such a call even if you didn't know that someone just put bugs all over your house.
Stupidity: When Brass and Sarah enter the locked storage cellar where the boys are hidden, Greg, a graduated rookie, straggles along behind them when he should have known better to stay put. As anyone could have come along and locked that cellar door behind them, trapping them inside.
Stupidity: Amazing how people on this show, like The Accused here and in the earlier episode "The Chef", are leaving their doors unlocked or no explanations given as to how the actual killer planted the evidence.