Also from Potterfandom we have Courting Miss Granger, about the Malfoy Marriage Curse. Said curse has been in force for several centuries, and dictates that the eldest male in every generation must marry a Witch — not necessarily pureblooded, but no more than one year older or younger than he is — before his twenty-sixth birthday, or the family fortune devolves onto a cadet branch and said eldest male will suffer a gruesome death. The Idiot Plot element comes into effect when NO ONE TELLS DRACO ABOUT THIS CURSE until his twenty-fifth birthday — when, conveniently enough, every Witch of his generation is conveniently married off ... except for one. And guess who that one might be?
For Your Eyes Only. The only way this AU society could exist is if everyone in it was insane.
The first case of Brendan Namron: Ace Attorney relies almost exclusively on Night Russ's stupid lies to drive the trial. The second case is much better, thankfully.
Eiga Sentai Scanranger has abundant examples, but probably the best is "There's Something About Toni." The villainess's plan is to have a hate-inducing monster attack the school's Valentine's Day dance, and have a student use pent-up energy to blow up the rangers at said dance. Some problems arise 1) She doesn't appear to take any steps to make sure what the student uses his energy to blow up is the rangers 2) She doesn't take any steps to insure that the student will even be at the dance 3) She continues on with her plan to use her monster at the dance, even though she earlier used it in a flashy attack that got the rangers' attention and allowed them to discover how to reverse the effects of its powers. This end up being okay for her because the rangers seemingly forget they have this information 4) The villainess was taken by surprise when she found out another superhero would be performing at the dance, even though this was advertised on the dance's posters 5) When the monster shoots an arrow at the guest superhero, one of the rangers tries to perform a body block to save her. Even though the ranger in question's signature weapon is a shield, and moreover a shield she's supposed to be able to throw with Captain America-like skill. 6) Not really part of the Idiot Plot but still worthy of mention, the rangers waited until after classes to do anything about the monster's rampaging victims during its first appearance. Meaning also that classes were held during rioting on campus.
The protagonists know that a girl will be raped by her stepfather when she returns home for Christmas? Instead of just keeping her at Hogwarts, they teach her needlessly complex spells and ship her home, so she gets her clothes ripped off before saving herself. A four-year-old and an infant won't eat the food being given, while locked in a dungeon? Two of the girls magically enlarge their breasts to provide milk. Possibly one of the dumbest moments is when Emily agrees to a bet with a Muggle girl named Rosaline. The terms are that if Jamie doesn't win a race, Emily must provide sexual favors for Rosaline. Keep in mind that Rosaline is seventeen, while Emily is eleven. Instead of refusing the bet (Rosaline gives the terms before Emily agrees) or going to Hermione or Harry to deal with a clear case of sexual harassment, Emily continues to panic in private, vomiting several times, before finally telling someone about it.
Not to mention Hermione's kidnapping, which wouldn't have happened if Hermione herself and the rest of the Hogwarts faculty hadn't somehow managed to get themselves outwitted by Crabbe and Goyle.
A relativity known Kung Fu Panda fanfiction called "A Bully's Return" has a example in Chapter 12. The epic battle that comes not too long after is put in motion when the main character goes to a weapons store to buy sais. After some bartering with the clerk, he lets the main character (which mind you is only 14 or 15 at the time) buy and leave the store with them. WITHOUT ANY QUESTIONS. AT ALL. Who in their right mind lets a 14 year old buy something that dangerous and think nothing is wrong? The bull clerk should be considered Too Dumb to Live.
Actually, a fourteen/fifteen year old buying sai in ancient china wouldn't be suspicious at all. A sai was originally a tool used to catch fish.
The notorious My Little Pony: Friendship Is MagicTroll FicPattycakes has this in spades. Sure, Dash. Just let your one chance to escape go rather than risk people seeing you in a nappy. Go ahead, Scoot, confront Psychoshy alone rather than get backup. Oh yeah, and everyone in Ponyville, feel free to indulge Fluttershy's growing obsession with mindraping everyone into submissionnote and this is the good interpretation - the alternative is that they're very much aware what they're doing and are simply evil.