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: 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: 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: 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?
The Return of Hydro-Man: Part 1 - S5-E7
Stupidity: When Spider-Man is working in the lab, he uses his webbing to barricade the door shut so no-one interrupts him. When Debra Whitman shows up and asks who is in there, he identifies himself as Peter Parker. When he is finished with his work, he leaves out of the window so that by the time the security guards break the door down, he has vanished. Since he used his webbing to keep the door closed, it would be pretty easy for Debra and the security guards to figure out that Peter is Spider-Man since Spider-Man's webbing is at the scene and Peter seemingly vanished into thin air.
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)
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 Promise - S6-E13
Stupidity: Markham lets Ava call her uncle's phone, and she doesn't let on that Boyd answers instead. He later says "I don't trust anyone", but he didn't demand she put the call on speaker for transparency, he's quite happy (at the time) to just hear one side of the call.
Episode #3.6 - S3-E6
Stupidity: The villain concocts a demented plan that puts him at absurd risk (if anything goes wrong, he is caught red-handed) and accomplishes nothing, since it does not make him and his accomplice unlikely suspects any more than anyone in the group. It does not make it look like it was an outsider doing it (no attempt is made to fake a robbery), nor a result of an accident. It makes obvious it was a murder (a knife plunged in the back!) but does not pin the guilt on anyone else in the team, which would have been really easy to do (plant the knife or any of the victim's belongings in anyone's tent) and with the two being in on the scheme without anyone suspecting they were connected to each other, it would have been easy to create an alibi for each other, but they don't have any. It should also be noted that there's no reason why the victim wouldn't have exposed the culprit's crime earlier to his fellow birders, since he knew he was going to harm the animals.
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.
Stupidity: The elaborate plan of the bad guy obviously involves driving Magnum increasingly crazy and create a rift between Higgins and him, but practically all the most damning episodes are outside the bad guy's control, and all the steps he takes are actually harmful towards him should have Magnum been in any way competent at his job. He couldn't have predicted that Magnum would paint the whole episode under the ridiculous light of "sumo wrestler and midget" instead of "big guy and small guy", he couldn't have predicted he'd stain himself, he'd try and force himself upon the girl, he wouldn't check the girl's background.
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).
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.
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)
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)