Stupidity: Dr Kepple is finally caught when he rushes to the hiding place of the calibration converter that he hid in the lamp after the murder. Why is it still there? It was a great hiding place initially when the police were searching people and the building in the hours after the murder but Kepple had plenty of opportunities after that to get rid of it. It is the only piece of evidence linking him to the crime and it is in his office so you would think that his main priority would be to dispose of it as soon as possible.
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?
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)
Episode #3.2 - S3-E2
Stupidity: Throughout the whole episode, no effort at all is made to find out where were the chocolates purchased, and the bottle of champagne that accompanied the box(es) is completely ignored, not even addressed in a throwaway line (such as that it's too common to trace, or whatnot). It's not directly part of the murder trick so it not even considered. The explanation of why the other incriminating box is still there is also quite ridiculous, since the murderer is left alone for enough time to do strike again with a very timely effort involving her leaving the only room of her bungalow (which involves her, the most known and conspicuous person on set, stalk her in plain view for an undisclosed amount of time), but could not find a few seconds to take a walk a few feet away and bury the box in the sand or bushes, flush the poisoned chocolates down a toilet, throw them at sea, just about anything. It's a bit of chocolate and cardboard, not exactly hard to dispose of that in a big set in the wild.
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.
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: 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: 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.
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.