Heart of Ice - S1-E3
Stupidity: Ferris Boyle tries to cover up his sabotage of Victor Fries' experiment, but he opts to store the incriminating security tape of the event in a filing cabinet instead of simply destroying the footage.
Stay Out Of The Basement (2) - S1-E13
Stupidity: When the real Dr. Brewer is found in the basement, he has duct tape over his mouth and needs his daughter to remove it for him. His hands are tied around the wrist, but not behind his back, so he could have easily removed the tape from his mouth himself long before he was found.
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.
Laura Ingalls Wilder (1) - S7-E1
Stupidity: Charles and Caroline are dancing at the church social. Charles makes a comment about Laura getting married. Caroline responds with "I'm just dying to call you Grandpa." Mary's son just died. Charles has already been a grandfather. That comment was beyond insensitive, and just plain stupid as written. She should've said, I can't wait to call you Grandpa again.
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.
Stupidity: After Cory and Shawn fish Mr. Feeny's watch from the septic tank, he suggests they jump into the lake to clean off. Cory protests, reasoning that it's only 20° outside, to which Feeny dismissively tells them to take a sweater. They then reluctantly abide. Someone of Feeny's education should know better than to allow them to jump into a freezing cold lake. They could both get hypothermia, and they're all currently isolated in the Pocono Mountains with no nearby medical facilities.
Snow White and the Seven Bradys - S5-E3
Stupidity: When the kids all pick the random-shuffled papers to choose which kid will be which dwarf, it NEVER occurred to them that with six kids and seven dwarfs they would be one short. This was only discovered once Peter (the last to pick one) was now up for a turn. How did nobody figure that out before the name-choosing even started?
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)
Long Night's Journey into Day - S5-E23
Stupidity: Rob tells Jerry "I haven't stopped sneezing long enough to take allergy pills" yet right after he says that, he doesn't sneeze anymore till he gives out his home address. He could have just taken his pills then. Someone behind the camera probably reminded him to continue sneezing. (00:16:52 - 00:18:00)
Suggested correction: He's exaggerating, but a character not taking a pill in this situation doesn't constitute a stupidly mistake (minor plot hole). I've been in situations where I needed to take an allergy or pain pill and didn't, even though I had a chance, because my mind or focus was somewhere else.
Stupidity: Aunt Bee learns that a New York writer and his wife have moved to Mayberry and she invites him to speak at a Literary Club meeting which will be held at her home. By telephone, Aunt Bee gives directions to the writer's wife. She says, "We're a quarter mile outside of town" - but she neglects to say in which direction.
Stupidity: All castaways knew about the highly potent drinks, but they each drank it all instead of just pretending to drink with fake sips each time Lord Beasley took a sip - thus knocking every one of the castaways out cold and losing another rescue.
Koopenstein - S1-E21
Stupidity: When Koopa walks slowly over to bros to capture the Princess, the crew just stand there. Toadstool, Toad, Mario, and Luigi make no attempt to avoid him.
Stupidity: When Richie and Fonzie swap clothing and walk in for The Duel with the Frenchman (both with faces covered to conceal their plan), the whole audience watching The Duel gasp and are all surprised to learn they switched places. But, before the switch was revealed, Richie (as Fonzie) walked in with a nerdier style walk and Fonzie (as Richie) walked "cooler" and looked much more in shape. Also, the head covering Fonzie wore was sheer and you could still see his face, but everyone was STILL fooled.
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)
The Ghost that Sacked The Quaterback - S1-E15
Stupidity: It's hard to believe that prior to building a new stadium the owner wouldn't remove all the trophies and memorabilia from the old stadium's sports hall.
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.
No Victory in Semi Final - S1-E21
Stupidity: The decisive goal happens merely because the players pass the ball to the opposition instead of clearing to the wings. Reading poorly the situation happens in real life as well, but here we have a group of 4 players who charge in close formation, and with all the rest of the field at their disposal, the two Nankatsu players pass the ball in the middle of them for no reason.