Dangerous Forbidden Technique

""By opening all eight gates, you could attain power beyond even the Hokages. The only drawback is... you die.""

- Kakashi on Rock Lee's Hidden Lotus technique, Naruto

There are some attacks in the media that come at a price. Actually, there are a lot of attacks in media that come at a price.

Perhaps the technique conducts a horrible toll on the person's body. Perhaps it's horribly immoral or damaging to the mind. Perhaps it calls up monsters that can't easily be controlled.

Whatever the case, the character's mentor will tell him that he can only use said attack a set number of times or he's never supposed to use it unless some requirement is met. Expect the Old Master to inform the hero that You Are Not Ready. And naturally, a situation will come up where the character has no choice but to use the technique, or if it has a limited number of times it can be used safely, he or she will be forced to use it more than the safe number of times. Like Forbidden Chekhov's Gun, this technique (or rather, its overuse) comes at a heavy price. Unlike Forbidden Chekhov's Gun, the price is nearly always paid.

If the hero is lucky, it could just be a Heroic RROD. But it's often worse than that.

Compare Deadly Upgrade, Cast From Hit Points, Cast From Lifespan. If they constantly survive these it's a case of I Thought It Was Forbidden. See also Godzilla Threshold and Lethal Harmless Powers.

Anime

 * Naruto has lots and lots and lots of these. In fact, there exists a specific subset of the Shinobi arts called 'Kinjutsu', literally "Forbidden Techniques". For one reason or another they have been banned from being taught, used, or both by the commanding shinobi. They are either jutsu that damage the user to a fatal extent (e.g. Mass Shadow Clone Technique), jutsu that are viewed as ethically wrong (such as Edo Tensei which resurrects the dead) or jutsu which involve other crimes (e.g. murder in case of the "Edo Tensei," because a Human Sacrifice is needed to resurrect someone). There also exists a wide range of techniques that are not officially 'kinjutsu' but fit this trope.
 * Sasuke's Chidori is only supposed to be used twice, but he uses it a third time; the result is that his cursed seal activates and nearly takes over (the technique later loses its "Dangerous" element, as Sasuke is able to spam the crap out of it without becoming exhausted). Just about every move Rock Lee uses that isn't a basic punch or kick carries some heavy toll on his body (such as his use of the Chakra Gates), and as a result, Gai forces prerequisites upon him, so that he must be protecting someone or defending his ninja way when he uses said moves.
 * In the second half of the manga, Naruto gains his Futon: Rasenshuriken, which is so powerful it actually causes widespread cellular damage in the arm he executes it with, which is far too minute for healing jutsus to repair. Tsunade even tells him not to use it again. After mastering Sage Mode, he perfects it so he can throw it, thus nullifying the risk to himself.
 * Lady Chiyo's clan dabbles with using chakra "strings" to control life-size puppets (and in some cases, assist a human by adding their reflexes to the subject's); they began work on a jutsu to truly breathe life into puppets, but stopped when the beta technique had a nasty side-effect of (you guessed it!) killing the user of the jutsu.
 * Hell, the deeper level of his Kyuubi form actually shortens his lifespan too. That's without considering the whole Super Powered Evil Side transformation.
 * As does Tsunade's own regeneration jutsu, which is consequently only used when there's a danger of a much more drastic shortening of her lifespan.
 * Pain has been shown to have
 * However, Nagato is a Uzumaki.
 * Don't forget the Mass Shadow Clone technique that Naruto learns at the very beginning of the series. It's forbidden because it requires so much chakra that the average ninja would kill themselves if they tried to use it, so it's restricted to use by Jounin level ninjas. Naruto, however, is a special case, since he actually has chakra capacity far beyond the level of most ninja, so he can use the technique with no ill side-effects. And to those who are wondering why Kakashi can use it, note that he uses the ordinary Shadow Clone technique. Only Naruto is shown to use the Mass Shadow Clone technique.
 * Not quite. There is no difference between the plain and "mass" shadow clone technique except the number of clones made. The shadow clone technique is forbidden. The problem with the technique is that it splits up the user's chakra between the clones, and some kinds of damage dealt to the clones are transferred back to the user. Kakashi used a mass Kage Bunshin after fighting with Zabuza, even though he was nearly out of chakra. He didn't need to use any chakra using techniques, and just hoped to intimidate some mobsters.
 * The Dead Demon Sealing Technique summons the fricking Grim Reaper to tear out your opponent's soul, and your own, and then forces them to fight forever in its stomach.
 * The recently revealed  allows users to cast genjutsu on themselves that can briefly override reality. As a tradeoff,.
 * The title character from Inuyasha can allow his demonic blood to temporarily dominate him by casting aside his mystical BFS, or having it stolen, which means it borders between this trope and Super Powered Evil Side. This "demon form" makes him a lot stronger but the more times he uses it, the more his soul decays. This may not sound like much, but what it actually means is that he becomes a berserk, vicious killer, and the more time he spends in this form, the more indiscriminate he becomes in choice of targets. It also becomes easier and easier to activate, and harder and harder to turn off... which means that he will, ultimately, become nothing but a mindless killing machine that can do nothing but wander aimlessly and slaughter everything in sight.
 * Rurouni Kenshin used this trope on both Shishio and Kenshin.
 * Hiten Mitsurugi Ryuu also has an ultimate technique that involves putting yourself in significant danger. If you hesitate even the slightest amount, you either die or lose your leg. If you don't hesitate, you might still lose your leg.
 * Sanosuke's Futae No Kiwami becomes one over time due to its overuse. His hand injury was said to be even worse than the damage Kenshin suffered against Shishio. Though his hand never really heals, he finds ways to minimize damage.
 * In Dragonball Z, Goku was taught the Kaio-Ken by King Kai, but was told not to use it at more than double capacity. When he fought Vegeta, he used it at triple capacity. When he fought Frieza, he used it multiplied by twenty -- the maximum, and still nowhere near enough to win.
 * The only time it really hurts him is when he pushes it to times four while fighting Vegeta, at which point he was apparently barely able to fight. The other times he uses it, apparently, his body's toughened enough to negate the negative effects.
 * Not quite. While his body was strong enough to withstand it after the Vegeta fight, when he used the 20x kaioken against Freiza, it ate away almost all his ki, leaving him defenseless.
 * He was also warned about the power of the Spirt Bomb when he is returning to Earth, particularly when the sun was out. The dangerous element of the Sprit Bomb is never mentioned again.
 * Another character, Tien, also had a forbidden technique: his Kikoho (Tri-Beam in the dub) will kill him if he uses it too much. Even when it doesn't kill him, it still exhausts him to the point of not being able to fight.
 * It's a circular blast fired from a triangle-shaped hand sign that leaves rectangular holes! Of course it's dangerous!
 * There's also the Mafuba (Evil Containment Wave in the dub), the technique that Master Mutaito used to seal King Piccolo. The attack requires so much energy that it kills the user, as Master Mutaito died from using, and later on in the series, Master Roshi dies after unsuccessfully using it against the now-released King Piccolo.
 * Vegeta's "Ultimate Final Skill" (suicide explosion). Of course, it doesn't work on Buu.
 * And that's not the only Senseless Sacrifice in DBZ; aside from the Saibaman who self-destructed and killed Yamcha early in the series, there were no successful kamikaze attacks. Let's see...Chiaotzu didn't make a scratch on Nappa, Kami was interrupted before he could finish (movie only), Android 16 had his bomb removed beforehand, and thanks to Goku, Cell couldn't destroy earth with his technique...though it's subverted with that last one due to his regenerative abilities.
 * In Eyeshield 21, Hiruma orders Sena to seal the "Devil Bat Ghost" technique. At first he believes this is to stop competition from seeing it, unless it is coming at them. But when he later breaks these orders, he discovers that because he is yet to master the technique fully, it puts tremendous strain on his knees, and repeated use could injure him.
 * The Devil Bat Dive is a lesser-used example, and for good reason. Using their try-for-point for a consecutive touchdown, the Devil Bat Dive requires him leaping over both lines, spinning, and falling into the end zone. Such a maneuver is dangerous in both fiction AND real life, which is why most teams just go for the kick less one of their players gets thrown into the ground head first.
 * In One Piece, during the Skypiea arc, there happens to a device called a Reject Dial, which is ten times stronger than the much more common Impact Dial. Using this has a large chance of destroying the user as well as his or her opponent. Wiper, A character intent on killing the Big Bad of that arc with the Reject Dial, uses the item thrice, and though his body does take massive damage, he escapes alive.
 * Luffy's ability to go Gear Second, by pressing his own rubber legs really hard to increase the blood flow around his body, upgraded his strength and speed dramatically. And you thought he was superhuman before. When he is first shown using it, he seemed very tired, and when he used it again against Rob Lucci, The Psycho for Hire not only deduced how the form works, but informed him that it's doing more than severely cutting his stamina, it's actually cutting into his life-force and could shorten his life span! Of course, this doesn't prevent Luffy from using the move twice in the same fight, and further more times against strong opponents. More so in the anime.
 * Although to be fair, the example from the anime was just filler.
 * Lucci even states that if it wasn't for his rubber body, the kind of stress Gear Second puts on Luffy's body would cause his heart to explode.
 * There is a possibility that Luffy has gained some resistance or outright lost most of the detractions of the technique after Kuma removed the accumulated damage. Post timeskip has yet to go into details though.
 * Monster Chopper also qualifies. While he does gain immense power, he not only loses his sense of self, but also harms himself due to the energy required to maintain the form.
 * Whenever Ichigo used the black getsuga tenshō in Bleach he weakened his hold on his body and allowed his inner hollow to take control. This was only recently averted when he mastered his hollow side, but there is a noticeable decline in his humanity every time he uses it in battle, resulting in Orihime being just as scared of him as his opponent when he used it in front of her in Hueco Mundo.
 * There's also Ishida's complete loss of his powers after his fight with Mayuri, during which he's forced to Do What Must Not Be Done.
 * Also of note is Yamamoto's use of a forbidden Kido in an attempt to kill Aizen. Hado #96, Itto Kaso (meaning "Single Blade Cremation"), uses the caster's own body as a catalyst to create a massive pillar of fire that erupts from the ground around the caster, taking the shape of a katana's tip. Aizen, unsurprisingly, survives (though by no means unscathed), but Yamamoto got better - rather than losing his life, as the spell had been implied to require, he only lost an arm.
 * In Simoun, the Emerald Ri Maajon plays this role.
 * In the latest chapter of Hunter X Hunter,
 * Yu-Gi-Oh: Industrial Illusions has the decidedly odd practice of designing cards "too powerful and dangerous to be used," requiring they be sealed somewhere or guarded to keep them from falling into the wrong hands. They always fall into the wrong hands.
 * In Yu-Gi-Oh GX, there's also something called the "Cyber Legacy", which Kaiser Ryō is the inheritor to. Normally, he and the other duelists in the Cyber-Style dojo (yes, the Legacy is apparently so powerful, you need to train in a dojo to properly duel with it) practice the normal Cyber-Style (Cyber Dragon and other Cyber-type monsters), but there's another set of cards known as the Cyberdark-Style that's sealed away from even the Legacy's heir due to its immense and dangerous power.
 * In the real game cards deemed too powerful are forbidden from official tournaments. If only they could be consistent on what overpowered means.
 * Ryoga Hibiki's "Shishi Hōkōdan" in Ranma 1/2. This Ki Attack is fueled by the user's depression and melancholy. So to acquire more power the user will purposely do things to become more depressed. When Cologne sees Ranma and Ryoga trying to out-depress each other while blasting the technique freely, she compares them to a pair of lions fighting as they roll off a precipice and to their deaths.
 * Surprisingly, Genma himself developed his own Dangerous Forbidden Technique: the Yamasenken and Umisenken, seriously and unusually (for this series) lethal techniques which equate the structure of a body with that of a house. Though initially developed, the techniques comprised by these two schools are designed for fun things such as ripping out an opponent's heart, tearing an opponent's throat, strangulation, and cutting up enemies into itty bitty pieces using nothing but air pressure. Genma was loath to teach Ranma anything about them, on the grounds that they were far too dangerous, and only acquiesced when he learned Ryu Kumon was using the Yamasenken. Even then, when Ranma and Ryu dueled using these two opposite schools, Ranma did so with the stipulation that, if he won, Ryu would seal the techniques forever.
 * The Shishi Hōkōdan is actually an interesting example on two accounts. First, Ranma tries and fails to beat Ryoga at his own game, and realizes the flaw in the principle-namely, as the tide of battle turns, the loser will gain great power for the technique by his depression at losing the fight while the winner's lightened spirits will depower the technique. So he creates his own, Not-So-Dangerous, Not-Actually-Forbidden Technique in the form of a Ki Attack fueled by boundless confidence. Second, Ryoga is baffled when the scroll he learns the technique from indicates a general sense of downwardness; it turns out that in it's purest form Shishi Hōkōdan isn't a directed beam, it's more like a weight dropping on the user from the heavens; the user is protected from his own attack because he's emotionally empty when he uses it. Ranma uses this against Ryoga by jolting him out of his reverie mid-attack with something he knows Ryoga wants from Akane; the surging hope and anticipation leave Ryoga open to being clobbered by his own technique.
 * Outlaw Star featured the Caster gun which fired spiritual Caster Bullets (each type represented by a number, but the differences were never fully established for most of them.) Eventually, Gene realizes that there are three numbers that never come up, so he tracks down the gunsmiths that make these bullets. Each one gives him one of these bullets (except for the #4 bullet, which he receives 2 of), but explain that they draw on the gunner's soul, so if he fires all three, he will die. Sure enough, he needs all three.
 * A slight inversion in the fact that these shells weren't originally designed to be harmful. It's just that there's so little mana left in the universe, that taking away lifeforce is the only thing powerful enough left.
 * Almost everybody in Get Backers: Ginji starts going crazy if in his Lightning Emperor mode for too long; Ban can only use his Jagan three times a day, once per person per day; Himiko's acceleration perfume strains her body; Juubei's Black Flying Needles are controlled by a large magnet, and the force of the magnetic field wreaks all kinds of havoc on his body and blinds him early on.
 * In Saint Seiya, Dragon Saint Shiryu was warned by his master never to push his fighting technique beyond a certain limit, lest he unleash the "Ultimate Dragon", basically sending himself and his opponent into orbit. Of course, Shiryu does end up unleashing the "Ultimate Dragon".
 * That being said, the technique was never used again in the manga. In the next arc of the anime, Shiryu attempted it again as a last resort move against an opponent that had beat everyone else, but stopped when doing so would destroy a MacGuffin that was needed.
 * Late in Tekkaman Blade, D-Boy finds that the Tekkaman transformation is slowly destroying his body. Later, he apparently gets better by "evolving" his transformation into the more powerful Blaster mode. Unfortunately, that turns out to be a better example of this, since now he's losing memories whenever he transforms, which for him is even worse.
 * The protagonist of Tsukihime has the Mystic Eyes of Death Perception, which allow him see the concept of death itself on everything in the form of lines and points. Observing death, however, will lead to insanity, so he has to use special glasses which block his ability. Every time he takes off these glasses his eyes get stronger, but since humans are not meant to observe the nature of death it puts an increasingly enormous strain on him. In fact, in the
 * Ryougi Shiki, who has a more outrageously powerful version of the Mystic Eyes of Death Perception, apparently isn't suffering any effects from using that ability. On top of that, she never uses any special glasses, even though she was offered them. It also helps that, unlike the Shiki of Tsukihime, Ryougi can turn hers on and off, while his is always on.
 * In the novels, her Mystic Eyes of Death Perception is always on. Though there doesn't seem to be a downside.
 * Mahou Sensei Negima's version of Black Magic seems to act like this. The first time we see it used is when Jack Rakan demonstrates it for Negi and nearly kills himself in the process. Negi then starts undergoing training so that he'll be able to use it safely.
 * And.
 * And the real direct consequence of Dark Magic finally shows up with
 * And even then, it's only a temporary fix.
 * In The Slayers, Lina's Giga Slave is the mother of all Dangerous Forbidden Techniques: It consists of, so not only will she die if it is miscast, but she'll take the entire universe with her.  Ragna Blade also counts, since it consumes magic quickly enough to put the caster's life in danger if they try to maintain it for too long.
 * Suffering from Dating Sim withdrawal, Keima of The World Only God Knows resorts to "Capturing God Mode" in order to clear his backlog, playing six games at once at blinding speed while still able to react emotionally to each one, and eventually passes out under the strain.
 * It's revealed in the third season of Magical Girl Lyrical Nanoha that Starlight Breaker causes considerable strain on Nanoha's body, and that her constant usage of it, as well as her tendency to use Deadly Upgrades, led to her being critically injured and hospitalized for nearly a year.
 * Actually, it was Nanoha's training regimen that resulted in her being low on energy during a battle, and the wounds she got very nearly killed her. However, the Deadly Upgrade that is the Blaster System defintely qualifies for this trope: if Nanoha's body is likened to a massive wooden building with fireplaces which supply magic to her, the Blaster System doesn't add fuel to the fire so much as it sets part of the building ablaze. In the following supplements, it's revealed that
 * It is, however, revealed that the Starlight Breaker compresses magic in such a way that when used the way Nanoha does it, compared to the way Reinforce used it, actually does put tremendous strain on her body, when combined with her training regimen, is what caused her to get critically injured.
 * Jin from Samurai Champloo learns his master's Dangerous Forbidden Technique only by word of mouth, along with (needless) explanation of its danger. During his final episode duel, he has to use it in battle.
 * Zoids: New Century has a mecha version: Among the three alternate armours available for the Liger Zero is the Panzer armour- effectively turning the Liger into a walking tank with huge guns and lots and lots and LOTS of missiles. However, it's so heavy and power-consuming that the Liger can barely move, and quickly overheats just from wearing the armour. The armour is still useful, but has to be ejected on the battlefield right after each use to avoid melting the Liger Zero.
 * In Soul Eater, using Tsubaki's mode takes a toll on Black☆Star's health. Early on, he can only maintain it for a short duration before passing out. At one point, the normally submissive Tsubaki refuses to use it out of fear for her partner's well-being.
 * Eventually, he finds a way around this, at which point the mode apparently stops having a dangerous effect. Really, all it needed was for Black Star to listen to others (namely Tsubaki) in order to have a chance of working out the technique, rather than assuming it took only physical strength. Being a decent guy really, he manages it.
 * Two notable examples exist in Sailor Moon: Sailor Pluto, who will die if she uses the power to stop time, and Sailor Saturn, who will die if she uses her power of destruction. They
 * In the anime there's also the whole Silver Crystal bit - use it at full power, which equates to a Kamehame Hadoken 9 times out of 10, and the user is killed. They get one last wish, though and it's usually enough to reverse it. This is not true of the manga.
 * In Yu Yu Hakusho, Hiei's Dragon of the Darkness Flame is an absurdly powerful technique that summons a dragon made of insanely hot hellfire that incinerates pretty much any enemy that comes into direct contact with it, even fire-resistant demons. The catch? If the user isn't sufficiently powerful enough, the dragon can turn on the caster and devour him.
 * The first time he used it, he also had to offer the use of his right arm as a sacrifice.
 * Train from Black Cat can only use railguns 5 times per day. In the final battle, he squeezed out 6th shot, resulting in him unable to perform it anymore. He's not worried, though, since he has already taken care of the Big Bad.
 * And Sven's eye ability makes him very tired after using it, which limits him as well, though his ability isn't near as badass.
 * Parodied in Ninin ga Shinobuden, where the technique that has been "banned ten years ago because it was so dangerous" is...duct tape underpants.
 * In Code Breaker, Toki Fujiwara's usage of his Gauss Cannon will damage whatever arm that he uses to fire. Thus meaning he can't fire more than twice overall in a fight. Every code breakers are in danger of suffering "after effects" when they overexceed usage of their powers.
 * The Experts in Giant Robo apparently have this. For some, using their powers to an extraordinary extent is this -- we get to see a mook burn himself to near-ashes with his pyrokinesis, trying to keep his buddies warm. For others, using their powers in excessive ways is this -- . And for at least one, using his power at all is this.
 * In Fullmetal Alchemist, human transmutation is the sole taboo when it comes to performing alchemy, and rightly so. The law of equivalent exchange is strict -- you must sacrifice equal or greater value, and the value of a human life places a horribly high toll on anyone who would dabble in such a practice. While trying to resurrect their mother, Edward lost a leg and Alphonse lost his entire body. It cost Edward his arm to merely tether Alphonse's soul to a suit of armor in a golem form through a blood seal. And that still wasn't enough for the alchemy to work.
 * Only two attacks are explicitly called thus in Saint Seiya: the Bronze Dragon Saint's "Rozan Kou Ryu Ha," which grasps the enemy from behind in a crushing bearhug and launches both user and target into outer space, where either air friction during the ascent or exposure to outer space will kill both people. Dragon Shiryu specifically noted how his master, Dohko, had banned this technique, and he only ever used it against the otherwise unbeatable Capricorn Gold Saint, Shura.
 * The other technique is the Athena Exclamation, a technique so devastating, so powerful, it has the power to annihilate the Earth. Therefore, it was declared taboo by Athena's Saints. It consists of three Gold Saints focusing all their Cosmo into a single point, discharging a blast with the same power as the Big Bang itself. Naturally, it was used in the final arc. And once that taboo was broken, it was used twice more --by two opposing trios of Gold Saints.
 * In Inazuma Eleven, has some hissatsu techniques, explicitly called "forbidden techniques", which are extremely powerful but strain the user's body; a single use is enough to cause searing pain, and three uses in a single match is liable to send the user to the hospital with the possibility of permanent injuries.
 * Prince of Tennis has the technique "Dash Hadoukyuu", Kawamura's most powerful shot . The original hadoukyuu already puts a great deal of strain on the arm, while the Dash Hadoukyuu is restricted to once per match..
 * The NT-D ("Newtype Destroyer") system in Gundam Unicorn. Built in to the eponymous Gundam, the system activates if it detects a Newtype (or cyber-Newtype) somewhere in the vicinity. It then releases all of the limiters on the Gundam's systems, directly linking all the suit's functions to the pilot's mind. This allows the pilot to gain extreme reflexes, almost mechanical precision, and even . However, this system can override the pilot's own morals with the system's programming to destroy Newtypes, and runs a very real risk of burning out the pilot's mind and causing severe brain damage.
 * In Bamboo Blade as well as real-life Kendo, the throat strike is treated this way, because of the high risk of seriously injuring your opponent if done incorrectly. In the series, it is used mostly by morally ambiguous or outright villainous characters, although the main protagonist Tamaki will also attempt to use it when sufficiently provoked or against an especially skilled opponent. In real-world Kendo, throat strikes are only permitted to be used only by high-ranking Kendoka.
 * In Raideen, the titular mecha has the powerful "God Voice" attack, which ends up ruining his vocal cords every time he does it. In the Super Robot Wars games, Akira will actually end up being unable to speak if you use this too many times before a certain point.
 * In Bamboo Blade as well as real-life Kendo, the throat strike is treated this way, because of the high risk of seriously injuring your opponent if done incorrectly. In the series, it is used mostly by morally ambiguous or outright villainous characters, although the main protagonist Tamaki will also attempt to use it when sufficiently provoked or against an especially skilled opponent. In real-world Kendo, throat strikes are only permitted to be used only by high-ranking Kendoka.
 * In Raideen, the titular mecha has the powerful "God Voice" attack, which ends up ruining his vocal cords every time he does it. In the Super Robot Wars games, Akira will actually end up being unable to speak if you use this too many times before a certain point.

Comic Books

 * In the Marvel universe, there exists a weapon called the Ultimate Nullifier which can Ret Gone anyone or anything the user chooses. But if the target isn't visualized perfectly by the user, then the user is the one who gets erased.
 * The "Demon Ball" technique in Bowling King; Its creator injured himself badly attempting to perfect it and was forced to retire from professional bowling.

Films
The entire point of The Dark Side in Star Wars.
 * Per The Expanded Universe, the lightsaber form Juyo/Vaapad, which is the most powerful of them all but relies on the heavy use of aggression. For that reason, it's forbidden to most Jedi except those who demonstrate they're capable of handling it safely (eg. Mace Windu).
 * In Kiss of the Dragon, Jet Li describes the titular technique as "very secret, very forbidden".
 * Though that technique actually represents no danger fot the user at all. What it does, however, is cause a slow, painful, messy death for the one it's used upon.
 * In Blades of Glory, the Iron Lotus is a technique developed by the coach in his "wild youth", but only one country was crazy enough to try it - North Korea. The only attempt resulted in the woman's head cut off by the man's ice skate (as impossible as this sounds). According to the coach, the only way for the technique to be performed successfully is by a pair of two men.

Literature

 * Death magic in Lois McMaster Bujold's Chalion series causes both the target and the caster to die as their souls are borne off to the Bastard's Hell (when it works at all). Not to mention that unsuccessful attempts/research are regarded as attempted murder at best and dealt with by temporal authorities as such.
 * The use of "balefire" in Robert Jordan's The Wheel of Time is forbidden by the magic users of the world, and when people use it anyway, they're exceedingly careful with it - because balefire doesn't just obliterate its target, it goes back in time to do so, with all the potential paradox that implies. Entire cities and their recent histories disappeared from the face of the earth before the mages, good and evil, decided that maybe gratuitous use of balefire wasn't such a good idea... When Rand begins using it for more than minute uses it is seen as a sign of his rapidly deteriorating mental state impacting his judgement, though he is technically correct that balefire is the only way to ensure enemies such as the Forsaken are not ressurected by the Dark One.
 * It wasn't just the cities disappearing that almost broke reality, but also having them reappear again as the destroyers were killed with balefire, and then having them vanish again if the person who did that also got killed with balefire. (Though Robert Jordan waffled on this a bit, at one point saying that the effects of balefire couldn't be undone by more balefire.)
 * Picking apart a weave of the One Power rather than leaving it to dissipate is a Dangerous Forbidden Technique among the Aes Sedai, and just a bloody dangerous technique among the Aiel Wise Ones. If executed perfectly it won't leave a telltale "residue," which normally lets one channeler see exactly what another has done, but failure is extremely easy and can produce a range of consequences.
 * And a lasting "fallout" effect that makes it difficult to cast spells in the entire region or a year or more.
 * Mercedes Lackey's Last Herald Mage trilogy mentions the "Final Strike", in which a mage uses up all his energy in an offensive spell, thus killing himself along with whatever is aimed at. It works, too, in the tragic ending of the trilogy.
 * It's also mentioned many other times in her other Valdemar works, and used at least once more on a very powerful scale. Where, in the Owl Knight trilogy  . In its own way, it's just as much a Dying Moment of Awesome.
 * Harry Potter has spells that count as Dangerous Forbidden Techniques, such as the Unforgivable Curses and Fiendfyre.
 * The use of Horcruxes is so forbidden that it's nearly impossible to find any information on how to use it.
 * Probably not least because it involves fracturing the soul, and doing that involves murdering people.
 * The Pendragon Adventure: It turns out that
 * In Anthony Reynolds's Warhammer 40000 novel Dark Apostle, creative thinking is viewed like this by the Mechanicus. Dangerous enough to be sealed away in a separate brain.
 * The Dragaera series has "Elder Sorcery", which involves the direct manipulation of raw chaos, and the practice of which is a capital crime by imperial edict. Those who research and practice it do so mainly out of curiosity and/or for the search for knowledge, since elder sorcery has long since been supplanted by the much safer and easier use "normal" sorcery, where the energy of raw chaos is first filtered through the Imperial Orb before being used. However, there are some circumstances where normal sorcery doesn't work, forcing the characters to resort to elder sorcery.
 * In The Magician's Nephew, the first chronological book in the Chronicles of Narnia series, there is a dark spell the Deplorable Word. When it is spoken, every form of life in the world with the sole exception of the speaker is killed instantly. Jadis actually boasts about the extensive measures she took to learn this and then use it to kill everybody in her homeworld as part of her backstory.
 * The Dresden Files: A wizard's death curse. Put simply, the wizard, usually when knowing she is about to die, draws up all the power stored in her body and mind and unleashes it in a powerful spell, usually a curse on whoever was killing her. Of course, this leaves no energy to keep the brain or heart working, so the wizard dies immediately on casting.
 * There's also Hellfire, which is demonic power that enhances spells but only available when your soul is corrupted by a Fallen Angel, and Soulfire, which is even more powerful than Hellfire, but actually uses your soul as the power source. Use too much Soulfire, and your soul literally evaporates into your spells, killing you.
 * In Garth Nix's Old Kingdom series (Sabriel, Lirael and Abhorsen), the last of the bells of the necromancer throws everyone that hears it deep into Death. What's it called? Astarael.

Live Action TV

 * Juken Sentai Gekiranger features the Ju-Ju-Zenshin-Hen, which allows the user to take on a super-powerful beast-like form. The bad news: transformation is permanent. The worse news: if your technique isn't perfect, your life's gonna suuuuuuck. A Wolf Fist user winds up transforming into an out-of-control werewolf periodically, but at least he gets restored into his human form. The Fly Fist user (no, really) isn't so lucky, winding up an anthropomorphic fly who is about the size of a mouse. He ends up getting swallowed by an evil Chameleon Fist user and is resurrected along with her after she dies...
 * Mahou Sentai Magiranger also has one. Anyone who uses Chronogel's forbidden time spell will die via getting sucked into a wormhole that appears on their chest while it sucks other things like matter and time into it. This curse was deliberately placed on it - apparently, the use of time magic is so dangerous that making sure the universe is screwed if you use it is the best way to protect it. Yeeeah.
 * Used on the main character in Eli Stone. When Eli needs to see the future with more clarity, he asks his acupuncturist Dr. Chen to use a little-known needle combination known as "The Dark Truth." After doing it once, Eli nearly has a heart attack, which makes Chen swear it off. So Eli winds up going to Chen's Distaff Counterpart and rival. He actually has the procedure done at least three times, which is probably why.
 * L's fiddling with the Death Note rules in the live-action movies could be considered this. There's also the more common in the anime world "technique" of Shinigami intentionally lengthening lives at the cost of their own.
 * In Supernatural, Sam's demon blood-drinking to power psychic abilities takes a toll on his sanity and turns him into an out-of-control addict. Since then, he went cold turkey (...with some exceptions...). In order to finish the Self Sacrifice Scheme to  Castiel also hints that 's current Meat Suit has to drink gallons of demon blood just to keep him in.
 * Kamen Rider Double's Twin Maximum, in which he activates two Maximum Drives at the same time, temporarily taking his power up to 200%. However, this puts an incredible strain on his body,
 * The bonehead maneuver in Babylon 5.

Professional Wrestling

 * Some moves can cause lasting damage to the performer's body. The two most glaring examples are the moonsault and the tombstone piledriver, both of which involve landing full force on the knees. The repeated stress of the latter move is why Kane switched finishers to the chokeslam and why the Undertaker developed the Last Ride.
 * Hulk Hogan with the Legdrop and Stone Cold Steve Austin with the Stone Cold Stunner. Landing directly on your ass hundreds of times a year must be great for the spine and neck (yup, they're connected.) But perhaps we should just cut this off right here and now and say that wrestling in general will pretty much fuck you up over time.
 * Most aerial techniques, but the 450 Splash and the Shooting Star Press are really bad, not only do they wear your body down overtime, but messing up could seriously hurt you on the spot.
 * The diving headbutt, and German Suplex (Which puts a lot of pressure on the spine), especially nasty since they were both moves that Chris Benoit used...a lot.
 * Technical wrestlers generally avoid this as most submissions don't actually put that much pressure on the body.

Tabletop RPG

 * In Dungeons and Dragons, there exist two magic staffs, the staff of power and the artefact staff of the magi which, while very powerful items in themselves, can be broken for a "retributive strike" which releases every spell inside the staff at once, centred on the caster. Given that the Squishy Wizard is the norm in D, anyone attempting this strategy had better hope that the 50% chance of getting sent to another dimension comes up.
 * There also exists an incredibly powerful dispel spell called Mordenkainen's Disjunction, which can destroy just about any magic effect, even those created by gods. If it is used for this purpose, however, the caster may permanently lose all his magic abilities and/or anger the effect's creator. And he is no slouch, believe us.
 * This is the point of the corrupt spells in the Book of Vile Darkness (and their Exalted Deeds counterparts, sacrifice spells). They are notably more powerful than other spells of their level--for instance, run-of-the-mill Blindness is a second-level spell, while Seething Eyebane, a corrupt first-level spell, causes the target's eyes to spew acid and explode--but there is always a tax, sometimes permanent, on one or more ability scores. The most powerful of these spells is harmful even to prepare and has a very good chance of rendering the caster dead and/or permanently insane (if he wasn't already).
 * In the Dragonlance setting, Wizards of High Sorcery view Primal/Wild Sorcery this way. It was three Sorcerers who, fighting an army of Dragons at the end of the Second Dragon War, caused magical storms that wracked Ansalon. These three Sorcerers ended up becoming the first Wizards after being taught High Sorcery by the gods of magic.
 * Several of the noble families from the Ravenloft supplement Legacy of the Blood possess the knowledge of a Dangerous Forbidden Technique or two, usually in the form of feats that only family members or their elite henchmen can select.
 * In the Mystara setting, Glantrians' use of the Radiance is considered this trope even by the Alphatians, who normally consider even the darkest sorts of magic to be permissible. This isn't because it's dangerous to the wielder (which it is), but because.
 * In the Sorcerer game from the Mage: The Ascension line, there is one Path that can have this effect: Cursing, which is Exactly What It Says On the Tin. Specifically, you can create an absolutely vicious curse rather than the normal version. The problem is, though, that it renders the caster a vegetable.
 * Genius: The Transgression has Deep Inspiration, which lets you draw out Mania directly from the Genius's mind, even if the Genius is "empty". Unfortunately, doing so too often or with too much power runs a very real risk of turning you into an Unmada, and if you keep pushing it even after becoming Unmada...Let's just say it's a bad idea.
 * Changeling: the Lost features Goblin Contracts, magical powers that are cheaper to buy than standard Contracts and have nice effects (open all the locks on a building, see the future, drain an enemy of all their Glamour). The catch? Well, they also have side effects that will likely screw you over (respectively, your locks fail the first time someone tries to break in, you go mad, you lose all your Glamour).
 * One particular Goblin Contract is "Call the Hunt" which has no real catch because it is its own catch; it calls forth a hunting party of True Fae. If the changeling who uses it doesn't run away fast enough, he or she will likely be killed or, even worse, dragged back to Arcadia to be tortured once again.
 * Plenty of Charms in Exalted come with heavy tolls. The more common include pushing up your Limit track, and Abyssals have some that increase resonance or mean that they'll experience Cessation of Existence upon death.
 * The Infernal Exalted get a good number of these, mainly because they're learning Charms that make them more like their Yozi patrons. Learn a Charm that gives you increased authority over lesser demons? That means greater demons are allowed to walk all over you. Learn a Charm that perfectly blocks Social attacks? That's because it turns all noise into wretched discord that makes you want to kill. Learn a Charm that allows you to communicate telepathically? Shame you can now only vocalize laughter for the rest of your days.
 * The Words of Power from GURPS: Thaumatology. Saying one will knock most characters unconscious and the most control you can ever have over a Word is none at all, trying to control it only makes things worse.
 * Scion has the Avatars, which allows Gods to channel the power of a Purview at the apex of cosmic power, so much that anyone who fights them has to become one in order to match. However, while the cost to activate it is rather modest game-wise, there are some major prices to be paid: first, you suffer the strongest Fatebinding upon activation, second, if you die, all the Avatars you learned to channel get loose all at once, and third, you activate it in the domain of a Titan, you'll attract every Titan Avatar in it, and chances are, they'll already have their own powers ready as well. Congrats, you now get to fight five or six beings with the same power as you.
 * Magic the Gathering has a bunch of cards that are effectively this. You can't miss them, because they all inform you that you lose the game after a certain amount of time or if a certain condition is met. For example, Final Fortune allows the user a free turn at a cost and color that doesn't normally get it, but the user loses the game at the end of that turn if they haven't won yet. Lich and its variants protect you from dying through life loss, but kill you under other circumstances, such as an empty graveyard.

Video Games

 * In Diablo III, the Wizard differs from the Sorcerer and Sorceress of the previous two games in that s/he practices "dangerous and forbidden magics", which eventually caused him/her to be voluntarily exiled from his/her homeland. This explains why the Wizard has a greater array of Elemental Powers than either of its predecessor classes, including Arcane and Time.
 * Similar to his anime counterpart, Shadow the Hedgehog can gain an extreme power boost whenever he removes the bracelets from his arms. Unlike the Sonic X version, however, it doesn't appear to drastically drain his energy. He only used this power once in Sonic the Hedgehog 2006 to save himself, Rouge and Omega from Mephiles the Dark, who used two of the Chaos Emeralds to create an army of clones of himself.
 * In the Fate scenario of Fate Stay Night, Saber comes under this limitation because using her Noble Phantasm requires all the mana she's currently holding to activate (and her flawed summoning means Shirou can't provide her with any), and she ends up facing at least three enemies that require -- or at least seem to require -- the use of it to defeat.
 * In Heaven's Feel, projection becomes this for Shirou. While it puts a strain on him in all three paths, this path spesifically gives him a clear limit on usage, and overuse will kill him.
 * Furthermore, each Servant has a power known as the Broken Phantasm -- willingly breaking their Noble Phantasm. This renders the servant without their proof of heroism (which for many of them is their weapon), but also inflicts massive one-time damage on whoever the Phantasm is shattered on.
 * In Final Fantasy IV, the sage Tellah is seeking the ultimate magic spell, Meteo(r), so that he can seek revenge on Golbez (the game's Big Bad) for the death of his daughter Anna. Sure enough, Tellah eventually learns the spell from his repressed memory, and uses it against Golbez, killing himself in the process. Other, more youthful mages, as well as an eternal precursor, can cast it without side effects. In gameplay terms, this means that he never ends up getting the required amount of MP to cast it.
 * In the DS remake, thanks to the New Game Plus mode the game has, you can easily subvert this: while the items to raise your max MP wouldn't normally be accessible until after his death, they can be carried over from your previous playthrough and indeed, using one on Tellah will allow him to cast Meteor as many times as you want with no ill effects.
 * Suikoden IV offers us the Rune of Punishment, which drains the user's life every time it's used. The rune itself is sentient, and tries to engineer events around it to guarantee it will keep getting used, until eventually its bearer is killed and the rune jumps to a new host, only to begin the cycle anew.
 * Interestingly, the Rune of Punishment governs atonement and forgiveness.  This, of course, implies that everyone that used the rune before was either a) stupid and greedy, or b) unable to understand the proper implications of forgiveness.
 * In their collective defense, Snowe was really, really, really hard to forgive. It's not so much that the previous bearers were lacking in forgiveness as it is that the hero has to be a veritable saint just to let Snowe live, much less invite him to join the crew.
 * And really, with the Rune influencing events so that, for instance, ... It's pretty much damned if you do, damned if you don't.
 * Compare the Soul Eater Rune in the first Suikoden game, which...eats souls.
 * Not exactly a Dangerous Forbidden Technique in and of itself, but, when the hero offers to use it to power up another rune towards the end of the game, he is severely chastised by Leknaat, who states that the true power of the Soul Eater must never be unleashed, implying this trope is in effect.
 * In the Megaman Battle Network games, the Dark Chips are extremely powerful, but that permanently reduce your max HP by 1 with each use. Also, each use drops your Karma Meter, and enough uses will disable Soul Unisons.
 * Battle Network 5 also has Chaos Soul Unisons, which allows you to use a Dark Chip as your charged shot for one round of battle without any of the permanent negative side effects. However, there is a 50-50 chance (game-breaking glitch notwithstanding) that the charged shot will fail and backfire, instead summoning an invincible shadow copy of Mega Man to join the enemies and attempt to beat the crap out of you.
 * In The World Ends With You, Joshua uses his
 * later proceeds to He reveals this himself to the player in the
 * Some Techniques and Spells in Romancing Sa Ga drain Life Energy if one is low leveled in That field of magic or if the weapon uses Life Energy for it's techniques.
 * Valkyrie Profile Covenant of the Plume has a heartbreaking example in the Destiny Plume. Left by Lenneth on Wylfred's father's body and corrupted by the goddess Hel, it can make any unit absurdly powerful - ten times as powerful in every single stat. And at the end of the stage, they die. Wyl is forced to use its power on his best friend and would-be Lancer at the beginning of the game...
 * Exactly how forbidden to make it is left up to the player, though, and affects the outcome of the plot.
 * Castlevania: Order of Ecclesia, blah blah blah, Dominus glyphs, blah blah blah, union kills you dead on the spot
 * To elaborate, Dominus Anger and Dominus Hatred are direct attack glyphs with each use inflicting shitloads of damage at cost to your HP, and Dominus Agony amps all your stats immensely with gradual HP loss thrown in on top of that. Unless you use a healing item at some point, prolonged uses of these glyphs by themselves will kill you. The Dominus glyph union kills everything in the area - Shanoa included save the aforementioned instance - dead on the spot.
 * Earlier in the series, Portrait of Ruin subverts the hell out of this. Pretty much for the entire first half of the game, you get warned about how having Johnathan or any other non-Belmont use the true power of the Vampire Killer will drain their life force and eventually kill them if they overuse it. But when you do actually unlock its power in game, you can whip it all day long with absolutely no consequences whatsoever.
 * Oh dude!
 * Uh yeah, that's because it takes longer than a day (and it's only a day they spend in Drac's castle), Johnathan shows no ill effects. It's implied that that will come later. He probably won't survive to see Drac's resurrection in 1999 if he keeps using the whip or doesn't somehow lock its power back up. It has also been said that using the whip too much will cause death, making it a possibility that he didn't use it well enough. Jonathan was urged not to over use it by various characters.
 * Breath of Fire: Dragon Quarter had an extreme version of this. The main character, Ryu, can transform into a ridiculously powerful dragon form at any time which can even floor bosses in a few attacks. However, there's a % counter in the top-right corner of the screen that's slowly ticking towards 100% throughout the game, and using dragon powers make the counter increase much more quickly than it normally does - and if the counter gets to 100%, it's game over. Without dragon powers, the boss fights are quite hard (especially the later ones), so it's down to the player to manage how often they use the dragon powers.
 * Vagrant Story has learned techniques that, when used, drain a portion of your health.
 * Akuma's (Gouki in the original Japanese) fighting-style from Street Fighter has the Shun Goku Satsu attack, which effectively drags both the target and the practitioner into hell, where they're assaulted by demons. Until it was mastered by Akuma, it was generally assumed that using it would always be lethal to both. Of course, in-game, it's just a move that does a lot of damage...
 * Team Fortress 2: There is an unlockable weapon for the Demoman class called The Eyelander, a massive sword which decapitates and heals its user on a killing blow. The downside is that having the sword in your loadout reduces your max HP by 25, because the blade feeds on your soul.
 * Averted when, upon killing enough people with the eyelander, the Demoman becomes second in max health only to the Heavy and in speed to the Scout (or a Soldier with the Equalizer at maximum effect).
 * A more appropriate example is the Equalizer. It's possible for a Soldier to become super fast and do huge damage when wielding the melee weapon and at low health. Obviously while powerful, using the weapon this way is very risky, as a casual shot could kill the soldier. Oftentimes cornered soldiers with no rockets use this tactic as a last resort.
 * A sillier example is the close combat Ullapool Caber. Is it a giant log that takes health of the user when you swing it? No. It's a stick grenade, that you whack people with. It only works once, and you sure as hell are going to feel it if you don't have any bonuses because it explodes in your face. Despite the fact that you are dead if you come across another enemy, it's quite deadly, and can be useful as a last resort or for OHK Os with a critical charge.
 * The first four .hack// games feature Data Drain, a technique that allows the user to either severely weaken a computer-controlled enemy, or Mind Rape a human being (most humans hit with a Data Drain end up in comas). The hero, Kite, is the only human with the ability. He uses it to weaken game enemies that have been hacked so as to have infinite HP, thereby making them defeatable, and to gather virus data to hack into protected areas of the game. The catch is that repeated use will corrupt his character data with the virus, causing nasty side effects in battle and, eventually, his character's death.
 * Multiple moves in the Pokémon franchise harm the user (such as Explosion), but these moves also tend to be very powerful.
 * And then there's the move "Struggle," which is a rather weak move that damages the user by 1/4th of it's max HP. Chances are, you're only using it four times before you die.
 * Focus Punch is extremely powerful, but at two costs. 1. It takes one turn to power up, leaving it open for attack. 2. If your pokemon is hit while powering up it "loses focus" and can't attack.
 * And, of course, the classic Hyper Beam and all of its variations. It has 150 base power, but also renders the user immobile on the next turn.
 * In Nethack, one can choose to break a magic wand in half, unleashing all the remaining power at once. Can be dangerous as most wands will simply explode.
 * In MLB Power Pros, Alvin has a special pitch, the Mirage Knuckler, but it is extremely dangerous to try to catch it, so much after Alvin and Mark, the catcher train with it for a week, Mark is covered with bruises.
 * In Valkyria Chronicles, Valkyrur are already so powerful that not only can they fire huge lasers, NORMAL BULLETS HAVE NO EFFECT ON THEM AND MORTAR ROUNDS CAN ONLY STUN THEM. However, they can sacrifice their life to do things on the scale of annihilating a fort, and the army inside it in a giant blue flame.
 * In Zettai Hero Project Dangerama's entire skillset is composed of these. As a Death Seeker, this makes sense.
 * The "Mortal Draw" in The Legend of Zelda: Twilight Princess. The attack involved sheathing your sword and untargeting the enemy. As soon as the enemy is within range you quickly pull out your sword and deliver a powerful attack able to one-shot most enemies. However, if you screw up the timing you're going to get hit.
 * Blood Magic in the Dragon Age games. It is extremely powerful and dangerous -- to enemies, allies, and the user -- and for many, many reasons, it is forbidden everywhere in the world. Even the Tevinter Imperium officially condemns Blood Magic, though in practice all of the Magisters are secretly Blood Mages.
 * Though the Grey Wardens don't forbid blood magic.
 * In Trauma Center: Under the Knife Derek's senior surgeon forbids him from using the Healing Touch after his first intentional use of it makes him collapse after the operation... with the caveat he's completely aware Derek will merrily ignore this instruction if he thinks the Healing Touch will mean the difference between life and death.
 * This is meant to communicate to the player that they should only use it as a last resort, as doing so will negatively impact their score. Good luck figuring that out without trial and error, though.
 * Dungeon Crawl Stone Soup has a lot of examples.
 * Lugonu's self-banish causes permanent damage to HP and MP.
 * As does Borgnjor's Revivification.
 * Most necromancy is partially cast from HP.
 * High-level summonings can break free and turn hostile at random, and those that can't will instead inflict nasty stuff like sickness and intelligence loss on the caster.
 * Downplayed with mid-level summonings, most of which have a chance to be hostile.

Webcomics

 * SEPULCHRI-TUUUUUUUUUDE!!
 * "No, you fool! Don't you realize if you initiate that attack, it will be the last thing you do??? Fiesta smacks some sense into you."
 * HADOKEN!!!!
 * Although not considered forbidden, Black Mage is only able to cast that spell once a day, and it makes a cute little nuke-sized crater wherever he aims it.
 * Spells that drain the net amount of love from the universe with each use and require the sacrifice of orphans to gain in the first place tend to have a bit of a social stigma against them. Black Mage seems to find an excuse to use it nearly every day...but then again, this is Black Mage we're talking about here.
 * Jigsaw Forte's Zombie Mode, while not explicitly a Dangerous Forbidden Technique, effectively becomes this when you realize if she uses it at all while on camera, she's blown her personal Masquerade (which means if she doesn't die from using it in the first place, she will when she's done).
 * Word of God implies that abusing the form does have plenty of consequences, but it depends on how she uses it / how much damage she sustains, not necessarily how often.
 * In El Goonish Shive, magic apparently drain users at various rates. Spells too powerful to handle may overtax even well-trained magic users, possibly even removing their magic for months at at a time.

Western Animation

 * Avatar: The Last Airbender: While it's not really forbidden, lightning redirection (a Firebending technique where you internalize lightning and shoot it in another direction) is extremely dangerous, to the point where even teachers of the move refuse to practice it with their students. First, you have to catch the lightning, which is just as dangerous as it sounds. While internalized, the lightning must be carefully controlled so it doesn't travel through vital organs. By the end of the series, only three characters know the technique, and only Iroh is skilled enough to avoid the dangers consistently.
 * Note that it's not dangerous, but it does fit the immoral aspect.
 * In WITCH, the Guardians can choose to transform into living embodiements of their elements, giving them Physical God levels of power- at the risk of losing their personalities and humanity, and being left open to mental domination from outside forces, such as Big Bad Nerissa..
 * Re Boot: Bob's fusion with Glitch is viewed as doing the unthinkable by Daemon's adviser, and Bob was fully aware that overusing his new Glitch powers would kill him. Bob comes very close to death when Daemon infects and forces him to overuse his powers to create portals. Bob does imply that the fusion would have been much less dangerous had Glitch not been damaged at the time (which is what necessitated the merger in the first place), however.
 * Transformers examples:
 * In Transformers Generation 1, Elita One turned out to have the power to, but as Alpha Trion had warned her, using it drained her Life Energy to a near-fatal level.
 * Also in Transformers Generation 1, Megatron has the ability to draw anti-matter from black holes and pretty much blow up all of his surroundings. It was used about twice in the Marvel comics, where it was explained he didn't often use it because he was liable to kill himself too.
 * G1 again, Windcharger can create magnetic fields capable of ripping apart even the strongest of metal structures. It burns him up very rapidly though.
 * In Transformers Cybertron, Vector Prime would
 * In Justice League until The Flash saves the day by running at extreme speeds (to the point where he was circling the world in mere seconds)  However, in the process he was almost swallowed by the Speed Force from moving so fast, and he says he probably won't be coming back if he ever goes that fast again.
 * In Justice League until The Flash saves the day by running at extreme speeds (to the point where he was circling the world in mere seconds)  However, in the process he was almost swallowed by the Speed Force from moving so fast, and he says he probably won't be coming back if he ever goes that fast again.

Real Life

 * The destruction of HMS Invincible at Jutland. The Dangerous Forbidden Technique in this case was the bypassing of safety protocols designed to prevent flashdown of a detonation in a turret from reaching the magazines, and it was done to increase rate of fire. But Invincible was pounding the crap out of the German Lutzow, so why not? Why not, indeed. The mist that was hiding her cleared, just long enough for two other German battlecruisers to get some solid hits in in the right place, and Invincible was blown in two. At least one and possibly both of the other British battlecruisers lost that day went up for the same reason.
 * In soccer, the awesome techniques of jumping up in the air and trying to hit a ball next to another player is usually forbidden, because it could hurt the other player if he'd get hit by a flying boot. A little more mild, but still similar is the "scissors" technique of scoring.
 * Nuclear weapons. Nukes are capable of great destruction, but actually using one comes at a hefty price. Large nuclear detonations release irradiated material into the surrounding environment that remains dangerous long after the explosion is over. In a more immediate sense, several nations have active nuclear weapons that are kept ready for launch at all times. Since there is currently no viable way to defend against a nuclear weapon after it's been launched, the target has no reason not to fire all its weapons at the aggressor. This is the concept of Mutual Assured Destruction (MAD,) and it effectively makes using nukes suicidal. Note that in World War II, this was not an issue; with no other nuclear-capable countries to provide the "Mutual" portion of MAD, the United States was free to attack Japan without fear of a similar retribution.
 * Nukes are literally forbidden by international law; even threatening to use nuclear weapons against another power constitus a war crime. Of course, some countries do still keep them anyway...just in case. On the other hand, Mutually Assured Destruction is actually now a defunct policy, because America and Russia decided that destroying the entire planet over even just a single nuke was even more insane than launching the nuke and taking that chance in the first place. Given how much damage a single nuke can do, and given that whoever uses it will face the wrath of the rest of the planet, this makes as much sense. Nowadays, anyway, the worry is more about the dangers of non-state actors (ie. terrorists) using them instead, which MAD would be powerless against at best.
 * This trope is usually very heavily subverted in real life, due to Combat Pragmatism being a time-honored military and general combative doctrine. If it's dangerous and highly lethal, using it is the best way to win. If it's forbidden, all the better, because that means the other guy will never see it coming.
 * Overdosing on caffeine as a deadline draws nearer.
 * Recreational drugs in general, actually. The more intense a drug's effects are, the more likely it is to cause harm if not moderated, along with the need for more strict moderation. And the more intense effects you experience, the more curious you'll get for even stronger experiences. Though just because an experience is weak doesn't mean the damage is light: air duster as an extreme example. Alcohol is an aversion, though: drinking more after getting buzzed actually decreases the euphoria and eventually can cause severe depression and rage.
 * Not exactly a technique, but human muscles are actually so strong that they can rip themselves off your bones. Usually body has all kinds of limitations on itself to prevent it from happening, like pain. But in situations where super strength is the meaning of life or death (like being slowly crushed under some heavy boulder), body can drop these limits by itself. It'll save your life when needed but at a really really painful cost. I wonder if it heals after that.
 * They can, so long as you're able to find a doctor who can properly set your broken bones and muscles so that the ligaments and tendons heal naturally. And, also, you'll need to follow his protocols for recovery to the letter.
 * See Limit Break.