The Mentor has a student with a great deal of natural talent. Unfortunately, that's a problem because all that talent is going to their head and risks turning them into an Arrogant Kung-Fu Guy or Insufferable Genius. Or it may just be that the student is failing to achieve their full potential because they have become too self-satisfied to put any real work into improving.
The student needs to be brought down a peg, but the student has progressed to the point where typical discipline like lectures or punishments won't do a great deal of long-term good.
To this end, the mentor decides on a different tack: The student needs to fail. The mentor thus arranges a situation where his big fish is taken from his little pond and thrown into the big ocean.
A more benign version of this can sometimes occur if the teacher is doing it less as a preventative or punitive measure, and more because they believe a legitimate defeat will be good for them in the long run. As the saying goes, you learn more in defeat than from success. It could also be done to encourage the student to accept the possibility of losing more easily than he might have otherwise. Or, depending on the instructor, less easily.
The effects on the student can vary. If the student is an antagonist, it may lead to Defeat Equals Friendship with him and the protagonists. He may learn nothing. Or it may backfire and drive him further into the dark side of life. If a protagonist, the student may indeed learn the lesson and grow stronger for it.
This could also lead to a break with the mentor if the student learns about the situation. Possibly even some Rage Against the Mentor.
Compare Unwinnable Training Simulation, which is often done with this trope in mind. The difference is that the Simulation is done in controlled circumstances while a Lesson in Defeat can be done in a real-life scenario. Subtrope of A Taste of Defeat.
- This is the primary reason for Master Roshi's assuming his "Jackie Chun" Secret Identity in the Strongest Under the Heavens Martial Arts Tournaments in Dragon Ball. He wanted to prevent Goku and Kuririn from getting big heads by progressing too far too quickly.
- Toraji, the Kendo Club instructor in Bamboo Blade sought this for Tamaki. He wanted her to discover a rival among her peers who could equal her in order to make her stronger in the long term.
- The spirit if not the specifics can be seen in One Piece when Dracule Mihawk defeats Zoro. Zoro definitely had the "big fish in a little pond" attitude when they met, though he was enough of a determinator to impress Mihawk. So rather than kill him, Mihawk defeated him thoroughly and encouraged him to see more of the world. Zoro's response to his crushing defeat was a tearful vow to never lose again.
- During the S-Rank Trials in Fairy Tail, Natsu went head to head with Gildarts, the single strongest man in the guild short of possibly their Master. Natsu put in a good effort, but when he got a taste of Gildarts's full power he fell to his knees and gave up on the spot. Gildarts passes him for this, since his intent was to determine whether Natsu had the judgement to Know When to Fold 'Em.
- In Buyuden, Isamu fell hard when Hayato defeated him, discovering that he needs to fight for real if he really wants to improve.
- Yomi from Yu Yu Hakusho had to teach his son/clone, Shura this lesson painfully. Shura had all the power of an S class demon, however his maturity level was matched his age. In order to prevent his son from becoming too wild and headstrong like he himself was in the past, Yomi saw it fit to give Shura this lesson very early in life.
- The main characters of Seraph of the End receive one of these at the hands of their commander, Guren, when he and his squad curb stomp them in a 3-on-5 mock battle. Their lack of discipline and teamwork is made clear when their formations are easily broken. Hot-Blooded Yuuichiro takes the lesson seriously and asks questions to make sure they don't repeat those same mistakes.
- Yuuma Kousaka in Gundam Build Fighters Try receives one of these at the hands of Tatsuya Yuuki. He challenges Yuuma to a fight, giving Yuuma his own gunpla, and using Yuuma's. The result is a Curb-Stomp Battle in which Yuuma realizes that the problem was never his gunpla, nor his own abilities, but his mentality during combat that was limiting him.
- When he was young, Toriko's Yuda wanted to inherit his master's shop, but his master refused. He explained to Yuda that, yes, he was a genius but because of that he'd yet to experience true failure and that would eventually make him overconfident. He ordered Yuda to go Walking the Earth until he understood that. Yuda learned this lesson when preparing medicinal food for a sick village. The only child willing to eat his bitter concoction died because there was a difference of one millimeter in preparing the food for a child compared to an adult.
- The main teaching style of Koro-sensei in Assassination Classroom. With every failed assassination attempt, he explained why the attempt failed so that the students could learn, plan, and relate the failures to their studies. Done by proxy with Karma during the first term finals. Koro-sensei allowed Karma to believe his natural intelligence would prevail where everyone else was cramming under Koro-sensei's tutoring. Karma proceeded to drop in overall class ranking but learned the lesson Koro-sensei hoped he would with the blow to his ego.
- In Usagi Yojimbo, little Usagi is given a lesson by his master. He's ordered grow some carrots from some seeds. Usagi cannot make them grow at all, and considers stealing some carrots from a neighboring farmer, but his honesty gets the better of him and he reports his failure, expecting to be thrown out. His master was testing Usagi's honesty; the seeds had been boiled and would never have sprouted.
- In The Empire Strikes Back, Yoda sends Luke into the cave to experience the dark side of the Force. He warns Luke that what he will find within is "Only what you take with you", but Luke disregards Yoda's advice that he does not need his weapons. When Luke enters the cave, he gets a vision where he confronts Darth Vader. After he rends Vader's head from his shoulders, the mask explodes, and Luke sees his own face underneath. The lesson to learn from this is that you are your own worst enemy, as well as a warning that not every challenge can be confronted with violence.
- Star Trek II: The Wrath of Khan. Kirk speaks of how, when he was in the Federation Academy, he found a way to beat the no-win scenario of the Kobayashi Maru test, something that gained him significant praise during his cadet years. Sure enough, no matter how difficult the trials he faced were, Kirk always found a way to win, proving that the test was invalid as far as his abilities were concerned and he could always win. In fact the first time that Kirk can't find a way to win is when Spock is forced to sacrifice his life to save the crew as there was nothing he could do. This proved that Kirk was wrong, you can't always win and save everyone; sometimes sacrifices have to be made.
- Referred to in The Avengers, when Captain America points out that Iron Man has never had a lesson in defeat... which makes him a less effective leader. It's debateable how true this is, given Tony's character arcs in the first two Iron Man films, especially the horrific Humble Pie he was force-fed when he was kidnapped by terrorists using his own weapons, but there's no way Cap could've known that.
- A Song of Ice and Fire: The final lesson to become a new maester is to sit in a dark room with a three glass candles, artifacts from Old Valyria made of twisted, sharp-edged obsidian. Many try to light them, but are only rewarded with bloody hands and wasted time. The wiser new maesters wait the night. The lesson is that, though maesters strive to understand the world, there are some mysteries that simply cannot be uncovered.
- This lesson is broken all to hell when Archmaester Marwyn is actually able to light the glass candles.
- Exile's Honor: Herald-Weaponsmaster Dethor is worried about several of his students because he can't give them this lesson due to age and arthritis. The Trainees in question got so many lessons in court fencing that they'd be toast the first time they ever came up against someone who wanted to kill them instead of score points against them. Once he recruits Alberich as Weaponsmaster Second, delivering these lessons becomes Alberich's primary duty.
- After Sam Vimes of the Discworld series becomes important to the workings of the city, and after repeatedly failing to kill him, the Assassin's Guild refuses to take contracts on him. They do, however, still send overconfident students on missions to the house he's had Crazy-Prepared with anti-assassin traps to see if they can get him in their sights.
- Many of his fellow students in Ender's Game think this is being invoked a couple of times against Ender himself by the teachers at Battle School and try to preemptively offer him consolation. It's never quite the case, and Ender always wins anyway much to his friends' amusement ("They had to cheat to beat you and they still LOST!")
- In Star Trek: The Next Generation A Rock and a Hard Place, this is part of reason why Commander Skamene wasn't fired after all his awesome actions. That man literally beat Kobyaashi Maru without cheating.
- Brotherhood of the Rose, by David Morrell. Saul and Chris start playing hookie from their orphanage so they can buy stuff from the outside world and sell it to the other kids. Their Parental Substitute Elliot (who has been secretly grooming them as future agents) sends CIA agents to beat them up and steal their money to motivate them to take up martial arts.
- In the Are You Afraid of the Dark? episode "The Tale of the Misfortune Cookie", David Lee is pursued by a warrior wearing dark armor after entering an Alternate Universe. After David tells the warrior that he just wants his old life back, the warrior slinks down in defeat telling David that his better half has won. When David removes the warrior's helmet, he discovers his own face underneath.
- The Kobayashi Maru test from Star Trek is the Unwinnable Training Simulation, designed to test how the students cope with a no-win situation.
- Doctor Who: Death Comes to Time includes a mission on a planet called Anima Persis that all Time Lords have to go through in their training, where they are meant to fail; in order to teach them what failure feels like so they know that even they are not infallible and the need to avoid the temptation to use their powers. It's a sort of humbling excercise. In the Doctor's words, "You take that memory with you wherever you go."
- In one episode of Xiaolin Showdown, Master Fung stated that there were a thousand lessons in defeat. He proves this point by beating the Chosen Ones in a wager by making it impossible for them to win, instead of fighting to win outright. Omi would use said lesson to prevent the villains from obtaining a powerful Shen Gong Wu. At the end of the episode, he challengers them to a second wager, which the heroes decline- They've learned they should "Never bet against Fung!"
- Done in X-Men: Evolution when the X-Men first fight against the Scarlet Witch who completely trounces them. While the team come out of it okay, they are a bit shaken by the encounter and Xavier muses this defeat may have been good for them in order to grow stronger.