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)
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)
Stupidity: With about 5 minutes left, the Black Sheep fly towards a Japanese carrier. They are flying Japanese planes. The Japanese spotter speaks in Japanese. The translation on the screen says "There ours" instead of "They're ours."
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: 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.
Girl Trouble / School Dance - S3-E18
Stupidity: Girl Trouble: Helga pulls a nasty prank on Arnold in class by gluing feathers to his bottom. Mr. Simmons sees this, but for whatever reason, does not give Helga detention for it. The next day, Helga spills paint on Arnold, then he throws some on her as revenge. Mr. Simmons sees this and gives Arnold detention even though he was simply fighting back at Helga for pushing him too far. Perhaps Mr. Simmons' brain was on vacation those two days. (00:03:16 - 00:08:42)
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.
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.
The New Original Wonder Woman - S1-E1
Stupidity: There's much fuss about who will take Steve Trevor back which is resolved via a contest. Why the Amazons don't heal him, if possible, then set him adrift to a safe port is never asked. They did this when Nazis invaded in the two part episode The Feminum Mystique.
Stupidity: While flying over the jungle in the Dragonfly, Dr. Quest uses the Dragonfly's speakers to tell the Po-Ho tribe he is their Air God and is angry for the tribe taking hostages. While he was at it, Quest should have ordered the Po-Ho's to release the hostages.
The Perks of Not Being a Wallflower - S1-E3
Stupidity: Superman finally smashes the weirdly strong bad guy through a wall, then goes to Lois, says "he's gone", and whisks Sarah to hospital. But he has no idea if the bad guy is "gone" or not - they've just had an epic fight, he has some sort of super strength / resilience, and for all Superman knows he'll come around in 10 seconds and finish the job of murdering Lois.
Suggested correction: Superman knows the 'strong bad guy' is close to his own strength. After fighting him, he knows what effect the crash through the wall would have had on him, if he was knocked out, and how long he would be out for. So he knew there was little risk of the guy coming around in 10 seconds. Alternatively, he X-ray scanned the area and saw the guy was no longer there.
Stupidity: During the scene at the restaurant, Rose pulls the fire alarm. Then she and the doctor run through the kitchen, past several chefs who just duck and crouch into corners. This would easily be one of the stupidest ideas ever. If there was a fire, they would evacuate out the back door instead of just sitting there waiting to be choked to death. And if they knew about Auton Mickey, running out the back door would also be a lot more logical than just crouching down and waiting to be battered to death.
It's Only Rock and Roll - S3-E11
Stupidity: When the gangsters show up at the house Pam and Dak Hampton are hiding in, Pam is captured. Dak then attempts to flee by running out the front door. What a dumb move. No surprise that he got captured as well.
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)
Stupidity: Some ancient advanced civilization took to the task of building this amazing net of stargates that enables instantaneous traveling over hundreds of light years or more, but any stargate can be disabled by anybody just by covering it with dirt or rocks. Apparently if the stargate is buried then it becomes inactive. See Season 1 EP.1.
Suggested correction: This is just you having a different opinion on how the Stargates should work. In fact, the ability to make a Stargate inactive by burying it is a brilliant move by the Ancients. Burying the gate, or even creating a barrier, prevents enemies (like the Goa'uld) from having a convenient way of invading a planet. Although some races, like The Nox, show the ability to activate a Stargate without having to form an unstable vortex.
Stupidity: The Resistance is a rescue team that helps the various protagonists escape schools. They do this by leaving intricate hints that lead to a detour to the school alarm instead of going straight there, setting off the alarm and rescuing the protagonists.
Stupidity: At the start of the episode, the Brunnen-G planet is about to be under attack by His Shadow in a big ship that is capable of destroying the surface of the planet and everybody on it in under a minute with its fire power. It seems the Brunnen-G had time to respond, seeing as they send a counter measure against His Shadow... Except that they only send a single squadron of 6 fighters. Their planet is about to be obliterated by His Shadow's killer ship, and they only send up 6 fighters. And of course, they fail and all die except for Kai. (00:01:45)