Stupidity: Ahsoka is fighting against the spear-wielding woman. She keeps blocking the spear using both her sabers. It would be very straightforward to block the spear with just one lightsaber (as she proves she can do just as well after she loses the smaller one) and use the second one to, for example, cut off the woman's hands.
Suggested correction: This is opinion, not stupidity.
Stupidity: Whilst Reva is running across the rooftops, there is a moment where she does a backflip in order to propel her forwards towards the next rooftop. If she was attempting to jump between roofs, then doing a backflip would not have been useful as it would only slow down her momentum instead of build it. (00:25:28)
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: 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.
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 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)
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: The master goes through all the trouble of making Luke the vessel so Luke can go and kill several people and risk getting the attention of the slayer, when all he had to do is have his minions go out and kidnap a bunch of people and bring them to him and he could've fed off them himself.
Storm Before the Calm: Part II - S1-E38
Stupidity: Shane could have easily avoided the blast from Lothor by simply flying away to dodge it. Instead, he just hovers over him and allows his Battlizer to be destroyed.
Day 4: 11:00 P.M.-12:00 A.M. - S4-E17
Stupidity: When the couple is in the plant they call Jack on the cellphone, he instructs them to open the case and separate the book from the activation device. This was stupid, he should have told them to destroy it, since the codes had changed the moment Air Force One crashed, and it would take about 1 hour to make this work. This is explained later on in the show.
Stupidity: Characters repeatedly make note of how hard to kill the Wraith are, but sometimes, especially in the final episode(s) of season 1, they go down easily in a few bullets each, and then don't even regenerate as the characters turn their attentions elsewhere.
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.
Black Butterfly Murder Case: File 1 - S3-E1
Stupidity: Since "Touno" was prominently featured on the photo of a lengthy article about the butterfly crypto-entomologist Shimon Madarame, it's quite strange to say the least that Kindaichi and his journalist friend wouldn't have the slightest clue about his name or generic occupation. Generally a magazine does not put a person in 1/3 of a double page spread without at least hinting the reader about a person's identity, especially when they are not doing anything in the picture.
Until Sunset, the Full Moon - S1-E7
Stupidity: This is not quite 'stupidity' but rather a "movie logic", supposedly sharp deduction that is really just a wild and nonsensical guess. Zenigata knows for a fact that Fujiko is impersonating Elena because she knew right away where to get the first aid kit, while Elena hasn't been in the mountain house since her husband's death. That makes no sense; Zenigata has to assume that Elena was completely straight about that fact (if it was the treasure's hiding place as he suspected, she easily could lie), and a person can easily remember where they store their security items even after years, for a variety of reasons.
Stupidity: Yoshi, Mario's sidekick who constantly follows him on dangerous adventures and has gone on dangerous adventures by himself, gets scared shitless when confronted by Riptor. This is not how Yoshi would act in a situation like this.