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.
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: Sir Topham Hatt is cross with both Percy and James. But why is James being blamed? He had no idea that Sir Topham Hatt didn't actually say that he had to stay in the shed and wait for him. James shouldn't have been blamed since he had thought Sir Topham Hatt actually said that and Percy should've been the only one blamed.
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.
Behold, a Distant Star - S2-E10
Stupidity: When Johnny finds out his father is alive and wanted for murder, he is livid with his sister for telling him neither fact. When Sue decides to tell him the whole story, she asks Reed to pull a photo album from the drawer next to him, which contains a newspaper clipping detailing that their father was wanted for murder. If Sue didn't want Johnny to know about that, she shouldn't have placed the clipping in a photo album, and then kept the photo album in a communal area where Johnny could have easily discovered it at any time. (00:06:19 - 00:07:34)
Beyond a Reasonable Scout - S5-E3
Stupidity: Phil the Scout ties a rope to Mikey's see-saw end's handle to lower Gus down without hurting him. Couldn't Mikey just climb off and hold the see-saw down with his hands and lowered Gus's end down slowly?
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.
Turkey Guys - S13-E5
Stupidity: The whole episode revolves on the turkey that Peter ate. Lois sends him and Brian out to get another one. She stalls dinner the whole episode waiting for them to get back. Problem is the turkey is frozen! It will take at least 6 or 7 hours to cook and Lois acts like as soon as they return with it, it won't take long to to cook, so the stalling makes no sense. (00:12:45)
Slave Island - S1-E7
Stupidity: The mutants that are held prisoner and forced to build the dam on Genosha are allowed to keep their costumes on. The prisoners are captured so that their powers can be used to build the dam faster, but there's no reason why they wouldn't all be given prison uniforms. Characters like Gambit and Jubilee are wearing jackets, and Jubilee uses it to her advantage by pocketing a piece of twine to pick the lock to her cell later on.
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.
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)
What If... The Watcher Broke His Oath? - S1-E9
Stupidity: Ultron says he can destroy galaxies with a thought. So regardless of Dr. Strange's protection spells, he could destroy the planet they're all standing on, or evaporate the atmosphere, leaving them all suffocating in space, and just step away from the fight entirely while they die.
Stupidity: Roberto Hongo is the center forward of the Brazil national team, and as we see from Tsubasa's poster/magazine, he did not alter his appearance except for wearing a fisherman hat sometimes. Despite being surrounded by football fanatics, he manages to keep a fairly low-key profile and it's totally not a big deal to have a top tier footballer hanging around the small prefecture.
Iron Man to the Second Power: Part 2 - S1-E10
Stupidity: When the other heroes are watching Iron Man battle his robot duplicate, Julia cheers him on and calls him Tony. This fight is taking place in full view of the public, and the public doesn't know that Tony Stark is Iron Man. They believe that Iron Man is Tony Stark's bodyguard. Julia could have easily revealed Iron Man's secret identity in this moment. (00:17:50)
Yomigaere! Eiyuu Densetsu - S1-E1
Stupidity: Despite being in training for 6 years at the temple to compete for a magical armor consecrated to a constellation and meant to harness the power of the Cosmos, Cassios and everyone else but Marin (Shaina towards the end) are mistified about the whole idea when Seiya uses it. What in the world have they all been training for, for 6 years, then? Shaina has to be the worst trainer ever. Cassios even calls it a 'superstition', but his master Shaina channels the power perfectly. It's like a Muggle working at Hogwarts and not believing in magic.
A Frightened Hound Meets Demons Underground - S1-E14
Stupidity: The bad guy is dumb enough to stay in the basement of Captain Sally's instead of taking his loot and fleeing.
The Ringer - S1-E1
Stupidity: As Link is watching Zelda from his tower, he's completely distracted and oblivious to the Moblins entering his room from the floor. They have the element of surprise and one sneaks up behind him. But rather than doing the smart thing and say, stabbing Link in the back or delivering a crushing blow from behind while Link is unaware of his presence, the Moblin simply just grabs him on the shoulder and puts his arms behind his back to hold him while the other Moblins go for the Triforce. Giving Link plenty of ability to kick one in the face, free himself and grab his weapon to fight them off. The Moblin should have just gone for the stealth kill, even if they aren't all that bright of an opponent. (00:02:10)