What An Idiot: My Little Pony: Friendship is Magic
Ever heard of the saying that "Friendship is the cure for stupidity?" Yeah, didn't think so.
"In Applebuck Season", Pinkie and Applejack attempt to bake muffins while Mr. and Mrs. Cake are not at Sugarcube Corner. Applejack, unfortunately is tired from apple bucking by herself. Her sleepiness causes her to not understand the ingredients and instructions, causing Applejack to substite potato chips for chocolate chips, regular soda for baking soda, a cup of sour lemon juice for a cup of flour, and earthworms for wheat germ. And that will end up not making a good muffin. You'd Expect: Pinkie Pie to notice the ingredients not being right (especially earthworms instead of wheat germs) and maybe tell the sleepy Applejack that something isn't right with the ingredients, and stop with baking the muffins, and start over (maybe with her baking the muffins by herself or with another pony that isn't tired). You'd have to be Too Dumb to Live to not notice any disgusting ingredients that aren't right. Instead: Pinkie doesn't even notice any of the ingredients being not right, and ends up baking the "Baked Bads", and ends up causing lots of ponies (including Pinkie herself), to get sick!
In The Return of Harmony, Part 2, continuing off of the previous episode, Discord has corrupted The Mane Six, except for Twilight Sparkle, he has also given back their wings and horns when declared that "the game is over". So Twilight can use her magic again. You'd Expect: Twilight to notice the fact that the color change of her friends turning grey as they get Discorded, use her regained magic abilities to try to fix the situation and try to restore her friends and uncorrupt them right away. Instead: While Twilight uses her magic to move certain objects, such as Rarity's "diamond" (which is actually a boulder), the Elements of Harmony necklaces and tiara (which is Twilight's) and flipping through her reference guide of the Elements of Harmony, she never even tries to use her magic abilities to restore her corrupted friends, and ends up continually getting worse, up to the point where Twilight loses hope and she gets discorded and turns greyand becomes severely depressed. If Celestia's letters hadn't shown up and Twilight didn't read them, Discord would have triumphed!
Also in Return of Harmony, Part 2, after Twilight restores her friends in the third act, she confronts Discord, who is ruling Ponyville in chaos. You'd Think: Discord would notice the fact the Mane 6 have regained their colors and have been restored to their actual elements and simply corrupt them again, saving him from being turned to stone again. Instead: While he does attempt to grab them psychically, he doesn't notice about their restored selves, gloats them in an unenthusiastic way saying "Fine, go ahead. Try to use your elements, frienemies. Just make it quick.", and suffers the consequences and turns back into stone.
In the episode Call of The Cutie, Apple Bloom is in class and Diamond Tiara passes a note to her. Cheriliee (the teacher) catches Apple Bloom and demands her to read the note. She sees the note on the floor and mentions that it’s blank. Diamond Tiara then retorts “Sound like anything else we know? [referring to Apple Bloom's blank flank]” The class laughs at Apple Bloom. You’d Expect: For Cherilee to scold Diamond Tiara for what she did Instead:She does nothing.
Another one occurs in "The Mysterious Mare Do Well", where Rainbow Dash becomes quite full of herself due to the attention she gets for her heroism. Her friends don't like how she's turning out. You'd Expect: They would have tried to talk to or reason with Rainbow Dash. Instead: The five of them start impersonating as Mare Do Well.\\
In "Ponyville Confidential," The Cutie Mark Crusaders' were revealed as Gabby Gums. You'd Expect: The girls would try to apologize to them in person, err in pony or at least defend themselves to the others that it was all Diamond Tiara's blackmailing them. Instead: They tried to get more stories from the pissed off ponies, causing the Power Trio to be treated like a pariah. Poor little fillies...
A moment after that, Diamond Tiara blackmails the Cutie Mark Crusaders because they attempted to stop writing hurtful stories. You'd Expect: For The Cutie Mark Crusaders to tell Cherilee what's going on to insure they can quit making hurtful stories. Instead: They don't, and are exposed as Gabby Gums later.
The second season finale, "A Canterlot Wedding", has several "What an Idiot" moments.
In Part 1, Twilight Sparkle goes to see the bride (and former babysitter) Princess Cadence isn't what she used to be, is acting really rude to her friends and family, and finally discovers something suspicious about her. You'd Expect: She'll gather clues to back her claim just like in "Mmmmystery On The Friendship Express" and go to Shining Armor or better yet, Princess Celestia privately to voice her suspicions and present her evidence, thus preventing any need for a spectacle. Or she could have at least call Princess Cadence out on how rude she is, that just because it's her wedding there's no reason to act like a Spoiled Brat. Instead: She straight out accuses her of being evil, causing Princess Cadence to cry. The result was that her brother gave a What the Hell, Hero? speech, her friends believing that Twilight was jealous, causing them to shun her, and her mentor became disappointed in her. Thus nearly losing her friendship and respect from them... if not for the fact that this Princess Cadence was fake.
In the same episode, Twilight's friends and family were unaware of why Princess Cadence was acting rude around them. You'd Then Expect: They would question themselves on whether or not Twilight is right about Cadence being evil or not when she said that, and would suddenly be suspicious of Cadence's behavior. Instead: They suddenly forgotten about how much Cadence had been acting like a jerk to them and straight up ignore Twilight anyway, with Shining Armor calling her out for her accusation. The end result? A Changeling invasion on Canterlot after Shining Armor's forcefield weakens. Needless to say, in an In-Universe example, Queen Chrysalis Lampshades this when she calls out the Mane Six on being too caught up in their wedding plans.
And then there's the fact that Canterlot received a threat. You'dThenExpect: Celestia to hold off any sort of event until they can take care of the problem, then invite Twilight and her friends to go and take out the threat when it does indeed arrive. Instead: Celestia decides to hold a wedding, but have the groom constantly cast a shield spell so massive that it's starting to wear on his mental capacity (it's mitigated by the fact that the bride gives him some energy boosts every now and then), increase security to ridiculous degrees (delayed letters and said shield), and then only telling Twilight about the wedding a day before it happens, not mentioning why they had to send it so late. This leads to Twilight looking like a Clingy Jealous Girl, which in turn led to the two other idiot moments above.
In Part 2, after Changelings successfully apprehend Twilight and her friends from getting to the Elements of Harmony, they are brought to the Changeling queen, Chrysalis. You'd Think: Queen Chrysalis would imprison Twilight and her friends in a cocoon just like Celestia, so that she would not able to free Cadence from the cocoon goop and break the mind control spell on Shining Armor. And then Chrysalis would win.The End. Instead: Chrysalis doesn't even bother trying to imprison Twilight, and she gets caught up in her Villain Song reprisal, allowing Twilight to free Cadence and have her be able to break the mind control spell on Shining Armor, and eventually Cadence and Shining Armor get Chrysalis and her Changelings blasting off again! Speaking of imprisonment...
After being alienated from her mentor, friends, and brother and even possibly had their relationship ruined forever. A remorsefulTwilight was spotted by Imposter Cadence/Queen Chrysalis. You'd Expect: Being Dangerously Genre Savvy she is, she could've lied to Twilight Sparkle, saying that everyone disowned her. Driving Twilight into Despair Event Horizon, and told to leave Canterlot at once. Granted, she could have been wary that Twilight might continue her search and find more evidence on her - but at least she could have teleported her somewhere that's NOT near the real Princess Cadence. Instead: She just had to play the Villain Ball and teleport her near the Canterlot Caves, giving Twilight a chance to discover the the real Princess Cadence. Also, what's with leaving the cave guarded by bridesmaids? Could she have picked less efficient guardians? Was brainwashing a few guards too much trouble?
And the above moment where Chrysalis lets Twilight free Cadence leads to even more idiocy, as Cadence then cures Shining Armor, and uses her love to power his spell. You'd Think: Chrysalis would realize that since Shining Armor's love was able to give her enough power to defeat Celestia, she'd try to stop them, imprison all the ponies, and then conquer Equestria unhindered. Instead: She says "What a lovely but ridiculous sentiment." What do you know, The Power of Love wins.
In Games Ponies Play, the Mane 6 are sent to greet the Equestrian Games Inspector. They don't have much of a physical description aside from her luggage. They find somepony matching that incredibly vague description. You'd Think: They'd at least greet her by her name. Instead: They don't, and spend the entire episode touring with some random mare. It's only by Deus ex Machina that the games inspector didn't get fed up by the random splashings and went to the exact same spa and talked to the same pony who got the greeting she was supposed to get that she based her decision to let the Equestria Games be there that year.
In the same episode, one of the Crystal Empire attendants gives a description of the Equestrian Games Inspector. You'd Think: In addition to name, they'd have a semi-detailed and useful description, like pony race, fur color, and Cutie Mark. Instead: They get a name and the description of her luggage. That's it. The first mare they see with that luggage, they immediately assume she's the Games Inspector.
In Magical Mystery Cure, Twilight receives an unfinished spellbook and is tasked with finishing the spell on the last page. You'd Think: Twilight would test the spell in a controlled environment to figure out what it did, then carefully work on fixing it to avoid any potential mishaps. Instead: She just up and casts the spell, thus switching her friends' cutie marks, and therefore their destinies in life.