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: After Cory and Shawn fish Mr. Feeny's watch from the septic tank, he suggests they jump into the lake to clean off. Cory protests, reasoning that it's only 20° outside, to which Feeny dismissively tells them to take a sweater. They then reluctantly abide. Someone of Feeny's education should know better than to allow them to jump into a freezing cold lake. They could both get hypothermia, and they're all currently isolated in the Pocono Mountains with no nearby medical facilities.
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: 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.
Free Family Portrait Studio - S8-E24
Stupidity: Dwight is seen walking out of the bathroom with the diaper in the direction of the main part of the office in which Angela sits. If he was to try to stay hidden (which was obviously the whole point), he would have gone through the exit of the annex, waited until she was distracted/or hide it for later and Angela never would have seen. This was done because the plot required for Angela to notice but Dwight could have easily kept it hidden. (00:10:40)
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.
Stupidity: Donna runs in to save everyone from the collapsing pylon, dying in the process, despite the fact that Superboy or Raven, both more powerful/resilient, could have stopped it falling without any issues whatsoever. Not even like anyone was distracted - the fight was over, they were all just standing around.
Stupidity: The super powerful Romulan commandos (no doubt with the help from the higher-ups infiltrated in the Federation) managed to touch up the footage of the fight in the roof so to remove every trace of the Romulan agents present. But Picard has been running, dodging blasts, ducking, and is knocked back by a huge explosion caught on camera and not deleted; we see it in this episode and it is unequivocally not caused by him. What is the point of going through subtly deleting every trace of the aggressors when it is transparent that something real happened and it's not all Picard's imagination? And the police simply take Picard back home from San Francisco to France without questioning him, not caring about investigating the explosion in the heart of the city involving a famous Admiral battling invisible enemies. If the conspiracy can erase anyone from the tape, why not erase Picard too, or the explosion and explain how he ended up K.O.'d differently?
Stupidity: After Dexter tells Hannah that the U.S. Marshals have distributed her picture everywhere, she questions how she will be able to flee Miami for Argentina. Dexter reasons he and Harrison can leave with her because the authorities won't be looking for a woman traveling with her "husband and child." It doesn't matter who she is or isn't traveling with. If the authorities have shared her picture, she's going to be flagged when she goes through security at the airport, which Dexter should certainly know. This is made all the more egregious when she takes Harrison to the hospital later in the episode and is ID'd by the receptionist.
Stupidity: When the new principal is addressing the student body, Avalanche tries to impress Kitty by causing a couple of small earthquakes, one of which causes the scoreboard to crash to the floor below. The principal laughs off the incident and continues to address everyone instead of either evacuating the area or having everyone take cover.
Stupidity: When Franklin and Tara are hooking up at the motel, it appears that sunlight is shining through the windows. Franklin is unaffected.
A Knight of the Seven Kingdoms - S8-E2
Stupidity: It is decided that those who cannot fight will hide in the crypt from the army of the dead, and Jon Snow suggests during the war council to put Bran in the crypt as well, "where it's safest." Jon knows full well that the Night King can resurrect the dead and add them to his army. Not only should he not suggest to hide Bran there, he should be telling everyone not to go anywhere near the crypt.
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)
Stupidity: Xander really has no reason not to believe Buffy when she tells him that Miss French is evil/a giant bug. There is an in-universe explanation of pheromones affecting him, but that's flimsy at best. He's already dealt with vampires and witches and body-swapping, and there has already been a murder around the time Miss French showed up... the fact he doesn't believe Buffy seems very forced and contrived.
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: When they are conferring before surgery everyone is keeping scrub except for Avery who has his arms crossed. Shortly afterwards you can see him look around at all his co-stars, realise he screwed up and try to casually raise his arms. (00:15:00)
Stupidity: Why do they always have to go through that washing machine setup in the laundromat when they have a great functioning elevator to the lab?
Suggested correction: Why would Gus go through the process of hiding the lab entrance stairs behind a washing machine but leave access to a freight elevator wide open? The elevator would be just as hidden and probably locked up. Not to mention it would be slower to take than simply walking downstairs.
Worlds Apart - S1-E2
Stupidity: The bad guy kills the deputy in the swamp, then later sees a little girl who witnesses the killing run away. He then searches the boat for a weapon and runs off after her after he finds a machete, without even considering the officer's gun still in its holster. Abby proceeds to make the exact same mistake later when she finds the dead officer (meaning there's danger nearby) and hears the girl screaming and runs in the direction of the screaming without taking the gun to protect herself.