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: 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.
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.
Stupidity: After eventually arriving at the stadium, why didn't Ash tell anyone that he was late due to getting kidnapped by Team Rocket? He arrived with half of his Pokémon exhausted and subsequently lost his battle. If he informed everyone about his situation, his battle with Ritchie would likely have been postponed until tomorrow, giving him some much-needed time to rest, which would have given him a bigger chance to win.
Stupidity: And the end, after sliding to another world, the group is caught on film by someone in her house. Quinn goes (illegally) into the home to get the tape. There is no point to this. Throughout the series, many people have seen them arrive, or leave, and usually when there's a mix up with one of their doubles, they spend so much time trying to convince people they are in fact from another world but can't prove it. Quinn doesn't even bother to see how long they're going to be on that world first.
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.
Halloween 2: The Ghost of Hetty's Past - S2-E5
Stupidity: If Sam was so worried about conserving oxygen, she should have blown out the candles in The Vault.
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.
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.
Lucifer, Stay. Good Devil. - S1-E2
Stupidity: It takes an awful lot of pointless visits and oblique thinking to simply investigate the known (and apparently only) collaborator and protege of the suspect, who also was on the crime scene. You'd think interrogating the business associate would be standard procedure.
Stupidity: When Franklin and Tara are hooking up at the motel, it appears that sunlight is shining through the windows. Franklin is unaffected.
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.
I'd Rather Twitch Than Fight - S3-E10
Stupidity: Like most dilemmas on the show, the controversy could have been resolved by witchcraft without Darrin knowing. Why didn't Samantha just twitch a duplicate tweed coat?
A Bad Reflection on You - S1-E38
Stupidity: The decision to risk revealing their identities to Bulk and Skull is one that the Rangers could have easily avoided. Since they tell the bullies to cover their eyes and ears, under the false pretense of a magic trick, they could have simply teleported to the Command Center, or any other secure location, and then morphed, well away from prying eyes.
Who, What, Where, Wendigo? - S2-E10
Stupidity: Dwight, Audrey, and Nathan all know Dwight's trouble is that all bullets are attracted to him, so why is he with them when they go looking for the killer, knowing every one is using a gun?
First Kiss - S1-E2
Stupidity: When Jeff Malone leaves the sandwich shop, he leaves the sandwich on the table. It would have been very easy for them to tell what was on the sandwich without needing to wait for Max to get home to recreate it. (00:02:41)
Living Hell - S1-E8
Stupidity: When the villain reveals to the doctor that he plans to transplant her brain into his own head, the doctor tells him it isn't possible because the brain can't survive outside the head. Forgetting to mention the fact that he will die the instant he takes his own brain out, making it rather difficult to complete the procedure, that is, if he didn't pass out from the pain of opening up his own skull first.
Firefly, Light My Fire - S1-E1
Stupidity: The button Benita uses to open her trap door also turns on/off the record player and swings the record arm.