Crisis on Earth-X, Part 3 - S4-E8
Stupidity: After the power cuts out, Thawne realises an obvious hiding spot and races to the secret room. He sees it's empty and just...stops looking? We literally just saw The Flash take a fraction of a second to scout out an entire enemy base we're told was 22km away! While he's dawdling in the secret room, Iris and Felicity get to Kara, knock out the guards, and get her down the corridor to the lift. He could have checked the entirety of Star Labs 10 times over in that time.
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: Damon and Stefan are trying to get Elena's humanity back - they could have just asked Rebekkah or Klaus or Elijah to compel her to turn it on.
Les Écorchés - S2-E7
Stupidity: The highly trained, elite security officer who tracks Angela down to the backups knows exactly what she is, how dangerous she is, and what she's capable of, but still drops his guard, flirts with her, lets her get up close and personal with him, and as a result gives her a clear window to grab his grenade to blow up herself, him, and all the backups.
Stupidity: Whenever General Grievous fights another lightsaber-user, there's almost always a moment with all the sabers crossed dramatically in front of both participants, one preventing the other striking a killing blow. Looks good, but utterly ridiculous, because he's got 4 arms and 4 lightsabers - he could easily block the blade in front of him with one or two arms, and simultaneously swipe at their head and legs, which no-one with two arms could ever defend against. Of course that would make for a lot of very short duels.
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.
The Book of Rebellion: Chapter Three: Angelitos Negros - S2-E10
Stupidity: The SWAT van transporting Khalil comes upon the most obvious setup in the history of setups, a crashed car with a miraculously unharmed woman lying in the street next to it, one arm suspiciously out of view. And of course one highly trained SWAT agent, instead of going in with backup, circling around for a proper look, or driving around (as his teammate suggests) or anything similar, wanders over and is immediately killed for his trouble. And then of course despite being surrounded by armed cops, she somehow manages to kill all of them without a problem (offscreen of course, because there's no way to believably show her single handedly killing about 20 cops).
Laura Ingalls Wilder (1) - S7-E1
Stupidity: Charles and Caroline are dancing at the church social. Charles makes a comment about Laura getting married. Caroline responds with "I'm just dying to call you Grandpa." Mary's son just died. Charles has already been a grandfather. That comment was beyond insensitive, and just plain stupid as written. She should've said, I can't wait to call you Grandpa again.
The Whole World Is Watching - S1-E4
Stupidity: The Dora Milaje arrive to take Zemo. John calmly attempts to talk them down, but it leads to a huge fight when he puts his hand on Ayo's shoulder. There are multiple instances during this fight where they totally would have stabbed John and Lamar with their spears, and are only stopped because Sam and Bucky intervene. While it's understandable they would want to obtain Zemo, attempting to kill two people to do so is completely out-of-character and goes against all of their morals and principles.
Suggested correction: I watched the scene and I never saw any of them try to straight up murder anyone. They were very skilled and precise in their attacks (for example, pinning John) and wouldn't have murdered anyone.
Here's a link to the scene (https://youtu.be/cpUq-kYMPkc). I counted three times when they almost kill them. First, after Ayo knocks John to the ground after he puts his hand on her shoulder, she attempts to stab him with her spear, prompting John to pick up the shield to stop himself from getting stabbed. There is no way she could have known that he would have picked up the shield, and if he did not do that, she would have totally stabbed him.
Second, there is another attempt where Ayo tries to stab John, which is only avoided because Bucky grabs onto the spear with his metal arm. Third, the two other Dora Milaje get into a fight with Lamar, and there is a point where one of them is about to throw her spear at him, but Sam grabs onto the back of the spear, so she cannot stab him. These are clear attempts to kill John and Lamar and were only avoided because Sam and Bucky intervened.
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 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?
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.
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 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: When Kellin is hit with the neurolytic emitter, it erases her memories of the outside world. However, the effects aren't immediate and take some time (enough to get her to sick bay and have time to talk to Chakotay). However, she spends her time telling Chakotay to help her remember their relationship, etc. when her memory is gone instead of just making a recording of herself describing the relationship and her feeling which can then be shown to her after her memory is gone. She might not believe the recording, but seeing herself discuss the situation would be better than just relying on Chakotay (who will soon be a stranger she never met) telling her something so unbelievable.