A challenge is overcome due to a character's specific quirk of insanity. Their inability to handle normal reality has been weaponized into an effective means of solving one or more problems. Keep in mind, this character's madness is essential to the task. A saner version would have failed.
This type of useful insanity is usually Played for Laughs. In Real Life we would expect these characters to get fired, arrested, and eventually hospitalized or killed. Not so in the world of fiction; these characters have been able to achieve things their saner counterparts are unable to, similar to Achievements in Ignorance. They're often as entertaining as a Bunny-Ears Lawyer, who is a capable character despite or alongside their quirks.
If their insanity prevents them from being killed or arrested when anyone else would be, that's a Lunatic Loophole. If a character's quirky but not actually insane, they're a Bunny-Ears Lawyer. If the bad thing that happened was so bizarre that they're the only person prepared for it, they're Crazy-Prepared. When all of humanity is composed of Heroic Madmen, that's Humanity Is Insane. If the crazy person was right about something, that's The Cuckoolander Was Right. If they gained any powers from their insanity, that's Power Born of Madness. When a crazy person is in a position of power, that's The Wonka.
Contrast Sanity Has Advantages.
- High School D×D: This is Issei's go-to strategy for a while, making use of Achievements in Ignorance to defy the odds and take his opponent off-guard to let his snowballing brute strength do the rest. A classic example is when he incorporates a fragment of Divine Dividing into the gauntlet of his Boosted Gear Scale Mail, merrily ignoring the fact this was supposed to be impossible. The trope actually gets examined as the series goes on and Issei gets Character Development - he eventually grows the self-awareness to realize these antics aren't without cost (the above sheered centuries off his lifespan) and if it ever fails, that's probably it for him. It forms part of his reasoning for training his head as well as his body.
- Jojos Bizarre Adventure: Joseph Joestar, snarky Pop-Cultured Badass and ostentatious Guile Hero has become well-known for the often nonsensical levels of trickery he uses to defeat his enemies.
- Neon Genesis Evangelion: Shinji Ikari certainly qualifies, since it's implied Unit 01's berserk mode is tied to his manic state.
- Paranoia Agent: "Radar Man" has been driven mad by the revelation, but his craziness also feeds him the information he needs to fight the menace to reality. Sort of.
- 52: All of Doctor Magnus's attempts to recreate the Metal Men fail while he's on his meds. Then he's abducted by people looking for mad scientists and, despite his warnings, deprived of those meds. He goes insane and, among other things, manages to recreate the Metal Man. Then he kills an egg-shaped supercomputer in the process of escaping. He did warn them he does crazy things without his meds...
- Continuity Comics: Crazyman, whose power is being crazy. "Crazyman" is recruited by a secret service as the "Plan B" to dangerous "unwinnable" situations. Danny is to the secret service as Monk is to the San Francisco Police Department, and comes up with solutions no sane man would come up with. If James Bond were insane, he'd be Crazyman.
- The Joker: A villainous example. A great deal of what the Joker does should be impossible. He has a tiny fraction of Batman's resources, doesn't have his own Charles Atlas Superpower to match, has a reputation that should make it impossible for him to secure allies and minions, and his scientific knowledge, while great, is limited to "realistic" inventions (unlike, say, Lex Luthor). And he's still one of the most successful (and terrifying) villains in the DC Universe, because insanity is his super power.
- The Sandman: One story has Delirium in a foul mood, having hidden herself away in her realm and cut off all access. In order to get to her, Dream recruits several mentally ill people, as only their flexible interpretations of reality would allow them to navigate Delirium's home unscathed.
- For a while, Sturmtruppen had the Drunk Captain, an officer drunk the whole time...and, being drunk, he'd mistake bullets from a machine gun for insects and swat them out of the air.
- The aptly-named Thor fan fiction Chaos War sees Asgard engaged in a losing war against the "shining ones", an Eldritch Abomination alien race who seem countless in number and devour everything in their path. Strategy is useless against them, as they always seem to be in the right place to subvert whatever Thor or Odin try, and the strength of the Aesir is waning. So Odin sends Thor to go get Loki and puts him on the throne, reasoning that if wisdom and strength have failed, madness might work instead. Loki promptly lives up to expectations, sending soldiers off to other planets at random — literally throwing darts to decide — and gleefully using his new power to indulge stupid whims like making an increasingly frustrated Thor collect him a bunch of acorns and closing down a highway to dry apples. Except everything Loki does works. The shining ones start showing up only on the planets where Asgard's soldiers are stationed, and the Aesir start winning battles with hardly a casualty. It gets to the point where Sif begins to wonder if Loki is somehow in command of the enemy, since he appears to be predicting their behavior perfectly despite his insane, random decisions. Finally Loki explains to Thor how the shining ones work: They can see the future. None of the Aesir's previous strategems have worked because the shining ones can see what's going to happen and adjust accordingly — but they can only make decisions based on what future they see at the moment. So Loki, as the enemy commander, has to mentally commit to a course of action that would benefit them, then change his mind once the shining ones are fully invested. To whit, he meditates on how much he hates Odin and Asgard and wants them to fall, decides to do horrible things like poisoning his own soldiers and letting the enemy invade Asgard while its undefended, and then when the shining ones show up to wreak destruction, decides not to do those horrible things after all and instead take advantage of the enemy's new position. Even the acorns and dried apples factor in! The mental gymnastics required are such that only a pretty crazy person could pull something like that off.
- The Infinite Loops contain a multiverse of characters like this. When you are an immortal time traveler who can gain new powers just by going to new places, part of the fun comes from seeing just how much insanity you can cause without causing a loop crash.
- Clopin from The Hunchback of Notre Dame. He has an oddly adversarial relationship with his hand puppet, and presides over a bizarro Kangaroo Court where he captures, tries, and nearly executes the movie's heroesall in the span of three minutes. And this guy is also the heroic leader of the rebellion against Frollo.
- Common joke among mathematics or computer science students and professors involves asking how many people became insane while studying in their departments. The correct answer is zero: insanity is a prerequisite.
- The main character's severe autism means he's the only one able to objectively synthesize all of the information and determine just what the deal is with the Starfish Aliens they discover. Of course, he's also unable to bond with other people or empathize with them, so...downer.
- The predatory instincts and complete sociopath nature of the ship's captain (who is an actual vampire) also turns out to be vital in second-guessing the otherwise indecipherable behavior of the alien vessel and its inhabitants.
- Jonathan Strange & Mr Norrell: The eponymous Strange must drink a bottle of distilled madness (causing him to lose his mind) in order to be able to see and interact with The Fair Folk on their own terms.
- Madness Has Its Place by Larry Niven: Jack Strather's megalomania and paranoid schizophrenia make him one of the few men left on Earth with the cojones to actually resist the invading Kzinti.
- "The Men Return" by Jack Vance: The short story featured a world where causality had basically gone out the window. One character was barely surviving, trying to find patterns where there were none. On the other hand, the ones who were crazy before The Event were basically gods.
- Never Let Me Sleep: Whether it's the way her brain naturally works, or the medication she takes for her disorders, Melissa is the only person not effected by the death sleep within the "South Dakota Quarantine Zone".
- Redwall: In High Rhulain, the hare Major Cuthbert went insane after his daughter was killed by vermin. He ended up killing a sea monster single handedly.
- This Alien Shore by C.S. Friedman: The Guerans (a mutant race descended from humanity) are all insane, which makes them the only ones who can pilot ships through hyperspace. In addition, the heroine's multiple personality disorder grants her a secret power that could alter the balance of power throughout known space.
- Deconstructed in Law & Order: Criminal Intent. Detective Robert Goren was encouraged by mentor Declan Gage to indulge his darker impulses to help him understand the criminal mind, and for a long time this approach served him well. However, after a particularly grueling season, he learned that Gage was manipulating events in order to kill Goren's brother and his nemesis, convinced that Goren would be a better detective with fewer people in his life to distract him. Instead, it drove Goren into a psychotic break.
- The premise of Monk is that Adrian Monk's obsessive-compulsive disorder allows him to notice details that normal people would miss, allowing him to solve crimes that stump the police.
- Seven Days: The only person able to pilot the Chronosphere is Frank Parker, inmate at a military insane asylum.
- Pretty much everyone in Farscape has moments of this, but John Crichton takes the cake, who has ejected himself into space and propelled himself to a nearby ship by shooting a gun into space and manages to force the Scarrans to negotiate with him by creating a handheld nuclear bomb which he promptly dances around with.
- Call of Cthulhu: There's a rule concerning "Insane Insight" that allows a PC going mad to obtain some hint from the GM as to what they're dealing with or the true natures of the cosmos.
- Bloodborne: Insight, a secondary currency similar to Humanity, represents your exposure to the Lovecraftian madness hidden behind the Gothic Horror premise. It is primarily gained by encountering eldritch bosses (and slaughtering them), consuming the memories of other madmen, and discovering how thoroughly infested and miniscule the world truly is. High levels of insight are typically dangerous; enemies gain new forms and attacks because you can perceive themnote , and you are more prone to the Frenzy status effect, which causes your blood itself to go into a frenzy and deal massive damage. However, Insight is useful for purchasing weapon-upgrading blood ores, and the madness imparted by Insight does have a specific use: without earning at least one point, you are incapable of realizing that the Doll is in fact supernaturally sapient, and is your only means of using your hard-earned blood points to level up.
- Dead Space: Isaac Clarke has the blueprints of the Marker in his head. This makes him the perfect guy to go about killing the source(s) of the Necromorphs, and allows him to read and decipher Marker texts. It also makes him paranoid, schizophrenic, and he often hallucinates dead people talking to him. Which is pretty par for the course for anyone who's survived close contact with a Marker, to be fair.
- DOOM (2016) and Doom Eternal: One of the things that makes the Doom Slayer such an effective killing machine is that he combines a certain blunt pragmatism with a deep, deep well of violent obsession; any man who finds his willpower restored by beating an enemy to death with their own arm has problems, but at the same time, this need to do as much damage to Hell as possible, as violently as possible, makes him willing to come up with solutions that a sane man would balk at, and perhaps need a change of underwear after hearing as a bonus. Best exemplified in Eternal, when he needs to gain access to a deep chamber on Mars that'll take too long to reach the old-fashioned way; he promptly hijacks a Wave-Motion Gun built into one of Mars's moons, blows a hole directly to the target area, and then gets access to this hole by shooting himself out of a cannon to where the escape pods are. The first part appalls Dr Hayden, who would be the Only Sane Man in a different universe, and the second even shocks VEGA, who has so far been down with basically any display of lunacy on the Slayer's part.
- Hatoful Boyfriend: Anghel Higure is trapped in a delusion that his life is some high-fantasy epic story and those around him are characters in the same. His fantasy stories are, in fact, a perfect metaphor for some of the other characters' personal storylines, including some very dark secrets about them that not even they know, the "spores" he rambles about are probably the biological superweapon samples the doctor has in the clinic, and his senses are highly sharpened by the paranoia of believing himself a fantastical antihero, enabling him to detect threats sooner even if he can't always communicate them.
- The New Order Last Days Of Europe: It is possible for the Siberian Black Army to pursue a nuclear weapons programme, either by making the research centralised, or by applying anarchist principles to weapons development. Yes, the SBA actually go door-to-door and ask everyone how they think a nuclear missile should work, and ask them to crowdfund it. Hilariously, this approach has a very good chance of succeeding - the resulting weapon is the Osvoboditel, or in English, "the Emancipator".
- Portal: Schizophrenic Doug Rattman was completely right in suspecting the operating system was out to kill everyone and so was the only survivor of the lab incident.
- The Secret World: Daimon is a firm believer in this, hence his astonishingly eccentric behaviour. As he explains in both dialogue options and cutscenes, he believes that only individuals who are utterly without structure, dignity and sanity can truly succeed in the dark days, and to this end has made himself as loose, adaptable and eccentric as possible. He even compares this to incidents where drunk-drivers somehow manage to survive accidents that would have killed sober motorists, reasoning that their survival is due to having lost all the self-respect that might have made them brittle, instead being malleable enough to walk away unharmed.
- In the Saints Row franchise, it becomes slowly apparent that pretty much every member of the Third Street Saints is insane to some degree or another, especially The Boss, and they use their craziness to establish their dominance among the criminal underworld in the most suicidally ostentatious ways possible and pretty much always come out on top. Their bizarre antics and ability to run circles around rival gangs cause the Saints to elevate from a street gang to a group of national celebrities, before then ascending to high-ranking positions in the US government until finally slaying an alien warlord, taking his species' vast technological advancements for themselves, and conquering the entire universe.
- Getting a good ending in Disco Elysium requires you to play your character like this; in the ending, it will turn out that the seemingly pointless fetch quests and idiotic political posturing that your character is getting distracted by all connect to the murder, with Kim pointing out the way that your unfocused approach allowed you to solve the mystery from all angles in the most incredible bit of detective work he's seen in his life.
- True Villains: Sebastian invokes this and doses himself with hallucinogens to defeat a Master of Illusion, so reality looks nonsensical to him, whereas anything normal-looking is being magically inserted into his mind.
- Darths & Droids: Out of character, Jim is apparently a very clever geophysicist, but he views roleplaying as a chance to "turn his brain off", which means that he rarely pays much attention, rambles incoherently on topics he's making up as he goes along, forms snap judgments that he rarely changes regardless of the evidence, says the first thing that pops into his head and otherwise kind of bumbles through the game in a sort of amiable fog...and as a result his characters come across as delusional, violent kleptomaniacs with no regard for human life and a five-second attention span. Those same characters, using the power of whatever ridiculous thing Jim just said, are powerful assets as soon as the Godzilla Threshold is crossed, because those absurd, short-sighted decisions either go right through the situation's weakness when rational thinking wouldn't, or crash into each other and lead to success in the chaos. That being said, nobody is letting him have a lightsaber again any time soon.
Pete: If only Jim were here.
Ben: So he could die too?
Pete: Everything's already totally screwed. This is exactly the time when his ideas work best. Everybody, think like Jim!
Ben: My brain just broke.
Pete: That's the spirit!
- Dr. Harold Pontiff Coomer from Half-Life but the AI is Self-Aware, who formed an underground boxing ring, has numerous clones that he kills to gain more power, punches the top off of a tank, kills aliens and mutants with his bare hands, and uses the highly dangerous Barnacles as a Grappling Gun.
- Red vs. Blue:
Epsilon: North was a crazy son of a bitch.
- Sarge, The Leader of Red Team. His plans generally make no logical sense, yet tend to bring surprisingly good results once he gets to execute them. Most notably, he caught Agent Washington at gunpoint by disguising himself as a cardboard imitation of himself, helped kill the Meta by tying a car to him and tossing it off of a frozen cliff, and made an adrenaline-fueled slow-motion car crash happen in real life.
- Freelancer Agent North Dakota, while more or less sane, is seen as following this trope In-Universe for his unconventional battle tactics such as using the upgrade that has 99.9% chance of killing him on the spot and pulling off a Guns Akimbo with sniper rifles.