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: 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)
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.
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.
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.
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.
Stupidity: During a raid of a Latin stash house, Hailey and Kim find two girls with a pistol. In Spanish, they say no bullets. Kim interprets what the girls say and Hailey replies, l didn't know you spoke Spanish. Makes no sense that a partner on an elite police unit in Chicago would not know a member of the team speaks Spanish. (00:14:58)
Stupidity: Ray is solo fighting the enemy. He lowers his rifle and grabs his sidearm and says last mag. He hadn't been in a prolonged gun fight. He used up his 3 mags on his plate carrier and one in his rifle. A soldiers typical combat load is seven mags, 210 rounds, but that's when re-supply is possible. For long range recons where they are operating far from the main force a Seal will typically pack 12 plus mags like the guys from Operation Red Wings since re-supply is not possible. This is just dumb. (00:38:40)
He Lokomaika'i Ka Manu O Kaiona - S8-E20
Stupidity: After being stripped of their weapons while looking for terrorists in the jungle, Steve and Catherine meet up with Jerry, who has not had his gear taken, but didn't bring a gun because he's only a consultant. Some time later, after they open some coconuts using a small machete Jerry had on him, they hear gunshots, and go over to investigate, finding two dead hunters. They take a pistol of one of the hunters right before encountering the terrorists. So far, so good. But then to fight the terrorists, Steve has Catherine provide cover fire with the handgun while he attacks them unarmed, and hands Jerry a rock when he asks what he's supposed to do. Why doesn't one of them take the machete? You can still see it on Jerry's back.
Stupidity: Andor and Luthen set a vehicle to drive and distract the security team. The vehicle is destroyed, and they immediately round the corner in full view of the team on a speeder and drive away, then blow up the vehicle. They could have driven away on any number of other streets without revealing that they weren't in the vehicle, keeping their escape secret. And if they blew it up before the team looked too closely they might have even been deemed to have been killed in the crash.
Yukiyasha Legend Murder Case: File 2 - S2-E15
Stupidity: A world-famous painter known to be anti-social dies in a plane crash with other passengers in Hokkaido, Japan. The first responders are 4 reporters, one of which recognizes the corpse at first glance. They bury the body and one of them, on the spot and with no preparation, decides to impersonate him pretending to have burn marks on his face and going on wearing a surgical mask and a wig for 10 years. This has so many logical flaws; for this plan to work, the police had to never ask him anything, check his health after being the sole survivor of an airline tragedy, and the painter needed to have absolutely nobody who'd recognize his voice or know him at all.
Stupidity: In the flashback, Emily's sister takes ill while playing her flute. In the next scene, the sister is sick in bed and Emily now has the morpher around her neck, but she is in the exact same outfit. If the sister was forced to pass the legacy on due to disease, it should have taken more than just a few minutes or even hours to learn that.
Stupidity: The last remaining Xindi, in desperation, attempts to release the virus over the fan, despite the virus not being fully ready. However, it seems the only reason he didn't release the virus at that time was he had to seek cover from Archer and T'Pol shooting at him and to start shooting back at them. However, he had plenty of cover from the rooftop stairwell, which was right next to the fan, and he had enough time to dump out the contents over the fan.
Episode #1.7 - S1-E7
Stupidity: When Eugene approaches Tsuda after Tsuda is shot by Ae-sin, Tsuda attempts to shoot him, but after pulling the trigger, he realises there are no bullets in the gun. Seconds later, a group of soldiers run up to him, and he attempts to shoot them, but he can't because the gun is still not loaded. Something he should have known since he only just attempted to shoot Eugene and failed because of it. (00:50:00 - 00:50:30)
When in Rome - S1-E3
Stupidity: A supposedly covert camera is placed in plain sight on a wall in a crowded room, with a prominent antenna showing, and a blinking blue light. (00:16:55)