Mr. Monk and the Astronaut - S4-E14
Stupidity: Steve Wagner is shown expertly constructing an elaborate hanging machine, but he adds a superfluous complication by having it triggered by a garage door remote control that he posts to the address in question. Anything could (and does) go wrong! Why doesn't he simply have it triggered by a bog standard timing device that you can buy in a hardware store for a tenner? I am not sure I'd trust him with my space shuttle.
Stupidity: Why would a cop bring up an old case when he knew he was the murderer? It makes no logical sense and only brought his downfall.
Suggested correction: He was being blackmailed by Gary Roberts. He "reopened" the case as a ruse to kill Roberts.
Stupidity: When Castle and Beckett discuss the trap LokSat set for them at the dead drop and any forthcoming retribution, Beckett comments that her dad should be relatively safe from reprisal because he's on business out of town. Someone as smart as Beckett would never make such brain-deadly dumb assumption; considering how powerful and how dangerous he's supposed to be, especially with the resources of the CIA at his disposal, LokSat surely would have no trouble finding and getting to, and certainly no qualms about eliminating, Beckett's dad if it stopped her in her pursuit of him.
Stupidity: The plan to infiltrate Emma into the coroner's office in a body bag is stupid. She is wearing a paramedic's uniform, and we see later when Mickey arrives, nobody is checking ID papers. How would she know that there was nobody in the room when she emerged from the body bag? If she could walk about the building without being stopped, why couldn't she simply walk in through the wide open front door in the first place?
Episode #3.1 - S3-E1
Stupidity: Ronan Murphy is left handed. Watch carefully as he disarms himself. His weapon is replaced in his right hand. In a suspicious shooting, they never thought to check something so basic? (00:04:12)
Stupidity: The whole episode is adorable 80s camp that makes zero sense. A very public attempt is made to the life of the star player, at a party and there are 3 bullet holes in a car to prove it. He hires someone to investigate, and the only way this guy has to protect him is to be added to the team. That's absurd; not just because it's not how pro football works, but also because being on the pitch getting physical with other players in no way helps to protect Darcey from a shooter that snipes at him from distance; Magnum would have done a better job watching him from the stands and being on the lookout at the hotel.
The Forecaster (No. 163) - S4-E10
Stupidity: Farnsworth tells Charnquist they've been compromised by a girl listening in on their phone calls and that Charnquist needs to take care of it. He is able to identify the girl, elude FBI, tries to burn his apartment down, is able to gains access to Maggie's apartment and knock out Aram in order to kidnap Maggie. He's a killer who works for someone who has arranged dozen of murders, and knows "take care of her" means kill her. There is absolutely no reason for Charnquist not to have just killed Maggie, Aram, and Fiona. He had opportunity and he ends up killing 3 cops in order to kidnap her and kidnapping Maggie would not have solved anything.
The Secret of the Flame Tree - S6-E2
Stupidity: Sylvie Baptiste is a successful writer, and she published novels after her masterpiece. The idea that she'd lose all her fame and fortune if someone published an academic paper making a completely unsubstantiated claim (if Sylvie does not have an original manuscript of the novel, neither does Lizzie) about her novel being the work of her crazy sister, is simply absurd. A good copyright lawyer or even any decent PR agency would put the appropriate spin to the accusations easily, and since her sister is certainly not gonna sue her and Sylvie is her only possible tutor, all those people supposedly very well connected with the academia and industry and shrewd in marketing their work, overreact for nothing. Even better; Humphrey has not even exposed the killer (or that it is a murder at all), and the university announces already that they plan to give a posthumous PhD to the deceased, for the paper she hasn't finished, based on an accusation with no evidence.
Stupidity: After talking to the bartender, Calleigh goes back the lab and talks to Caine, and he wonders if they got a false positive from the vic's blood alcohol. Calleigh then gives a detailed description of how that could happen; Caine already knows how (obviously this was done for the audience, to explain it, but in the context of the show, it makes no sense).
Stupidity: When Mac and Lindsay are reconstructing the stab wound found on their victim on their pig specimen, a couple of the weapons they use are long, thin knives; since they obviously don't match the circular wound, Mac at the very least would know those could be excluded by default.
The Succubus (a.k.a. Chef's Mama) - S3-E3
Stupidity: When Kenny and Cartman are sitting on the bench, Cartman can't start the tape as he has fallen asleep. Kenny then tries to wake him up, but wouldn't it have made far more sense for him to just start it himself, as he is sitting right next to it? Cartman's job was to push the play button. It was not like he had to rewind, fast forward or even eject the tape, so all Kenny had to do was just simply push the play button and tape would have started to play.
Will the Real Martian Please Stand Up - S2-E28
Stupidity: The whole plot is based upon 6 people not being able to spot the seventh (odd man out). We can eliminate the couples, the driver remembers the pretty blond getting on the bus and the crazy old guy would stick in anybodies memory, so it leaves only the sour old guy in the coat and hat as the martian.Also all the troopers has to do was ask for IDs which they do but quit after they ask the crazy old guy for his.
The Secret of the Unicorn: Part 2 - S1-E4
Stupidity: When the Bird brothers are searching the treasure room for Tintin, one of them claims they have found him, and points. The other brother then shoots at him which was turns out to have just been a mirror reflection. The brothers then proceed to find Tintin and start by going straight forward, completely in a different direction than the mirror. If they wanted to find Tintin, wouldn't the more logical thing to do be simply look in the direction the mirror was pointing at? The whole point of a mirror is to reflect what is in front of it, so Tintin would probably have been standing in that direction anyway. (00:37:25)
Stupidity: Dexter's decision not to kill Trinity when he had the chance eventually led to Rita's death, a mistake Dexter told Deb he would never make again. However, at the end of this episode, he decides to spare Oliver Saxon so Deb can arrest him. Dexter doesn't stay to ensure that Deb successfully has Saxon in custody, which leads to Saxon escaping his shackles with the unwitting help of Deputy Marshal Clayton, and shooting Deb before leaving the scene. Dexter therefore does make the same mistake again.
God Complex - S5-E23
Stupidity: Maggie and OA are searching the building. As they walk up the stairs, Maggie is using the Harries technique, but she has both her handheld flashlight and the gun's flashlight on at the same time. This defeats the purpose of the technique. Additionally, it's daytime, so the building is lit up from natural light sources, but they still have their flashlights on. (00:13:36)
Stupidity: When the bad guys are discussing the plans inside a private jet, the scene finishes and there's a shot of the plane in the sky. The plane immediately veers to the right, and spins almost 90°, as fighter jets do when they turn quickly. If there were three people inside, (one of whom was in the bathroom) they would have been thrown about the cabin quite violently. (00:11:25)
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?