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: Each Shadowclone takes a portion of the shinobi's chakra for himself. Make too many Shadowclones, and the original will probably die from exhaustion), 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.
Sasuke gained the ability to use the Chidori more often by the growth of his chakra capacity. Kakashi has likewise increased the number of times he can use his own forbidden techniques, Raikiri (an enhanced Chidori) and Kamui (which sucks anything he looks at into another dimension). As Kakashi explained to Sasuke, when dealing with chakra-heavy jutsu it's essential for a ninja to know their limit. Attempting to exceed that limit will most likely cause the jutsu to fizzle, and the user will suffer potentially lethal chakra exhaustion. Thus, when Sasuke tried to use that third Chidori in a single day, if he didn't have the cursed seal it probably would've resulted in a Senseless Sacrifice.
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. While Naruto's Healing Factor can still repair the damage on its own, having to heal from something that extensive still reduces his lifespan. 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. Chiyo winds up using it to save Kazekage Gaara (yes, the same Gaara that houses a demon, not unlike the titular Naruto).
Hell, the deeper level of his Kyuubi form actually shortens his lifespan too, as it involves repeatedly burning away and healing back Naruto's skin. That's without considering the whole Super-Powered Evil Side transformation. Eventually, Naruto befriends the Kyuubi, allowing him to access its full power without physically transforming and thus without any risks involved.
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 an incredibly powerful version of his gravity attack that takes off years of his life, but can destroy an entire city in one blast. And he uses it.
However, Nagato is an Uzumaki, which means he has his clan's longevity, reducing the risk of Shinra Tensei's big version.
The recently revealed Izanagi allows users to cast genjutsu on themselves that can briefly override reality. As a tradeoff, the eyes used to cast the genjutsu will close forever.
Finally, there is Edo Tensei, a jutsu that sacrifices a living person to bring another back from the dead as a zombie with unlimited regenerative abilities. It is forbidden both because of being necromancy and because it is ridiculously broken - the undead not only regenerate from ashes as long as their soul is in place, allowing them to spam their own dangerous forbidden techniques with impunity, but since the chakra reserve is tied to physical stamina, the regeneration also keeps replenishing their energy. When it is used to its full potential by Kabuto, the resulting Boss Rush takes thousands of ninjas to stop.
The risk of Edo Tensei is if an exceptionally powerful Edo Tensei, like Madara, knows the contract, they can break it. Thus even if the summoner cancels the technique, the Tensei would continue to exist...now free of the control.
Quoted above is the Eight Inner Gates, a technique that grants exponentially greater amounts of strength each time one is opened, but with more and more strain on the user's body. Rock Lee, before the timeskip, was almost permanently crippled by using 5 gates and having his arm and leg crushed by Gaara. Might Guy, Lee's teacher, is stronger and more able to use it, but it's indicated that if he opens the eighth gate, death is guaranteed even for him.
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 Superpowered 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. The full body-bandaged Shishio would and did die from heat exhaustion after 15 minutes. Kenshin was forced to retire from swordmanship for good because of his overuse of the Hiten Mitsurugi Ryuu, as his fighting style is meant for more muscular men.
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 Dragon Ball 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 had to use it at triple capacity, which hurt like hell, and then quadruple during the Beam-O-War - after that, Yajirobe just patting him on the back made him scream in agony. By the time he fought Frieza, he could use the lower levels without any strain (King Kai had to tell the others that he'd been using it the whole time and wasn't holding back). He eventually used it multiplied by twenty — the maximum, and still nowhere near enough to win. That wiped out his reserves and left him nearly defenseless.
He was also warned about the power of the Spirt Bomb before returning to Earth, particularly if he planned to draw power of a star.
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.
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, in games it's called Final Explosion, which reduces you to a single pixel of life). 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.
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 enough to curb-stomp Kumadori, he not only loses his sense of self, but also harms himself due to the energy required to maintain the form.
Not anymore: He can now keep control of himself while in this form, though once it wears out he's left pretty much helpless for a while.
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. He got better.
Ichigo's "Final Getsuga Tenshou" has the side effect of causing him to lose his powers completely. That's why it's called "final".This might also explain Ichigo's father's power loss (he only started recovering them a few story arcs back).
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 the latest chapter of Hunter × Hunter, Gon forced himself to grow by shortening his lifespan, using all his inborn talent to reach the power that rivals the King of Ants (strongest character as of now), with the possibility of never being able to use nen ever again (and having lifespan shortened)
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. (It's little wonder why the game is such Serious Business in this series.)
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. Ryo learns just why it's so dangerous and forbidden the hard way.
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 as tools for burglary and thievery, 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. And when he did fire his last shell, he did drop dead; so did his enemy. He was saved because the place he was in (the Galactic Leyline) noticed everyone inside was dead and performed an Auto Revive.
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". But he doesn't die. His opponent has a last-minute change of heart and manages to send him back to Earth with just a kick. Seriously.
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 epilogue, which takes place a while after any of the routes, it is revealed in his reunion with his 'sensei' that he is literally on the verge of death, in part due to his ability; it is heavily implied that he actually dies just after they part ways.
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.
that's because she has the help of a 3rd personality doing the observing nature of death bit without frying her brain for her which Shiki Tohno doesn't have he has to use his human brain which isn't made for this.
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. In Chapter 249, we see the first sign of the possible consequence of using Magia Erebia.
And chapter 265, we see just why this could become a problem.
And the real direct consequence of Dark Magic finally shows up with Negi basically overdosing on magic and poisoning himself, which is now requiring him to have to battle inside himself to get an upper hand on the dark magic. Eva isn't even sure this is possible because as she noted earlier, Negi has already surpassed her in his mastery of this technique. Oh, and even if he wins he apparently won't be exactly human anymore.
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 summoning a fragment of Lord Of Nightmares into the physical plane, so not only will she die if it is miscast, but she'll take the entire universe with her. Naturally, it's miscast, and only a literal Deus ex Machina on the part of the Lord of Nightmares saves the day. 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 God Only 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.
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 her use of the Blaster System during the JS incident permanently decreased her magical potential by about eight percent.
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. He allows himself to be stabbed, and takes advantage of his opponent's position at his side and effective defenselessness (his sword is stuck in Jin) to strike him down.
In Soul Eater, using Tsubaki's Uncanny Sword 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 both break the taboo in the manga anyway and actually do die — in Pluto's case for a whole arc — before coming back fairly nonchalant and well-adjusted about the whole thing.
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. Also subverted, as the one moment the attack is turned on Hiei, he's become powerful enough to control it, not only surviving, but also temporarily absorbing it into his body, becoming much, much more powerful.
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 — Alberto the Shockwave breaks apart after absorbing the Monster Sphere's energy field, for example. 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, Teikoku 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 until Final Hadoukyuu. The original Hadoukyuu already puts a great deal of strain on the arm, while the Dash Hadoukyuu is restricted to once per match. Naturally, he breaks this rule against Gin Ishida.
There are quite a few others as well: Tezuka Phantom and Zero Shiki Serve, Ryoma's Cyclone Smash, Atobe's Tannhauser Serve, Sanada's Rai...
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 the ability to hijack Psy-Commu weapons, which are generally a Newtype's most dangerous weapons. 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.
You wouldn't expect this from an anime about the circus, but the arcplot of Kaleido Star revolved around one of these. There was a Dangerous Forbidden Technique for trapeze and highwire artists that was so dangerous and forbidden, Fool refused to even tell Sora what it was because as soon as she heard about it, she would be so obsessed with it that she'd try it even when she wasn't ready, and die.
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. This is possibly the only weapon that can truly defeat Galactus. (Possibly. He is known to be afraid of it, at least.)
The "Demon Ball" technique in Bowling King; Its creator injured himself badly attempting to perfect it and was forced to retire from professional bowling.
In the Buffy the Vampire Slayer prequel comics, there are dark magicks that the Watchers are forbidden to use. Someone does it anyway, and gets revealed by Giles. But Giles had also used a forbidden technique to reveal the guy, which is why he gets put through a torturous test that either drove everyone else who took it insane or killed them outright.
Naruto's original technique, the Blood Clone Jutsu uses blood to form clones almost as tough as the creator, that return any leftover chakra to the original when the technique ends. But the amount of blood required means that anyone without a healing factor would pass out creating more than one Blood Clone. Naruto creates a stronger version that has Sakura and Ino heal him and give him plasma pills so he can make a small army of blood clones.
Breaking the Souhi is not unlike the retributive strike below, it (according to lore anyway) instantly reduces the temperature of everything in a five kilometer radius to -100 degrees (Fahrenheit or Centigrade not specified), including whoever breaks it.
In Kiss of the Dragon, Jet Li describes the titular technique as "very secret, very forbidden". (Though that technique actually poses no danger to the user. It is forbidden because of its effect on the target.)
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.
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). Luckily for some so affected, not only is the Bastard's Hell is not only more of A Hell of a Time than a Fire and Brimstone Hell, but they don't always stay there, as some of the other gods may take them up. Not to mention that unsuccessful attempts/research are regarded as attempted murder at best and dealt with by temporal authorities as such.
Of course, successful attempts at death magic are considered divine intervention, and not prosecutable under criminal law. This is a bit of a semantic point, since if successful there's usually nobody left to prosecute. Usually.
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. When Elayne tries and fails, it produces a burst of wind. A burst of wind with the force of a small nuclear explosion.
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 Vanyel's already massive power results in a Final Strike that is so literally earth-shaking that the geography of northern Valdemar is forever altered, in the Owl Knight trilogy poor old almost-powerless Wizard Justyn must expend his Final Strike to destroy the single bridge that stands between a marauding barbarian tribe and his fleeing village. 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.
Ironically, despite the fact that there was supposedly no known countercurse or any means of defending yourself from Avada Kedavra, Harry Potter survived it three times in his life, two of which were direct hits. Voldemort survived it once as well (for the same reason Harry did the third time, so someone was clearly misinformed).
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.
Similarly, the drinking of unicorns' blood grants one life, even when one is all but dead, but at a terrible cost: that one lives a half life, a cursed life, from the moment the blood touches their lips.
The Pendragon Adventure: It turns out that all Travelers can pretty much warp reality like Saint Dane can, but doing so drains the life force of Solara. Saint Dane avoids these drawbacks by relying on a warped version of Solara based on darkness.
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.
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.
Hellfire, which is demonic power that enhances spells but only available when your soul is corrupted by a Fallen Angel. Soulfire, Hellfire's divine counterpart, actually uses the stuff that composes your soul as the power source. Use too much Soulfire, and your soul literally evaporates into your spells, killing you (and maybe then some).
In Garth Nix's Old Kingdom series, the last of the bells of the necromancer throws everyone that hears it deep into Death, including whoever rings it. There is nevertheless a point in the series where the situation is desperate enough for this to seem like a good idea.
In Perdido Street Station, Isaac relates to Yagharek how a previous administration's Torque experiments were banned once pictures of what that utterly-malignant energy had done to a rival city went public.
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 his aneurysm gets worse by the end of Season 2.
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. It gets worse when it's revealed all he did was just so he'd be tricked into releasing Lucifer. Since then, he went cold turkey (...with some exceptions...). In order to finish the Self-Sacrifice Scheme to let Lucifer possess him so he can jump into Lucifer's prison and prevent the world from being razed, Castiel says this requires for him to drink A LOT of blood. Castiel also hints that Lucifer'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, as demonstrated when Shotaro impulsively uses it in one battle, which lights him on fire and severely injures him. In the final battle of the series, Double's Super Mode is powerful enough that he can combine the Xtreme and Prism Memories' Maximum Drives without ill effect.
The bonehead maneuver in Babylon 5: Opening a jump gate within an existing jump gate, resulting in a massive explosion of energy that the ship who triggered it is highly unlikely to survive or outrun. The heroes only use it because it was the only way they could think of to shake the Shadow ship trying to kill them.
Resurrection spells in Buffy the Vampire Slayer. They either turn the dead person into a zombie, or make them come back wrong and all screwed up.
The heart removal technique for vampires was in the Angel half. Most vamps won't go near it because they don't know how long they'll be invincible for, only that it could be minutes,hours or days,and then they die.
In Merlin, using the Cup of Life to become immortal. The result is that the person who drinks from it becomes the living dead of a sort, not a zombie, but tainted still.
Nova blasts from Bionicle, during which Toa characters unleash all of their Elemental Powers in a massive explosion. Not only does it leave the Toa with no power left, depending on the type of element it can also easily kill anyone in a not-so-close vicinity.
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.
Speaking of the piledriver, an actual piledriver (not the tombstone version that the Brothers of Destruction use) is something that WWE superstars haven't been allowed to use for over a decade due to Owen Hart using the move on Steve Austin, giving him the neck injury that would eventually end his career (no offense to Owen). Come 2013, the fact that Punk used it all, let alone on Cena (who's had a history of neck problems), upped the Holy Shit Quotient of the match they were in. Said match is now considered one of the best TV matches in RAW history.
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. Just ask Brock Lesnar.
Booker T's Harlem Hangover (a top-rope flipping Guillotine Legdrop) combined the wear and tear of aerial moves and legdrops. Back problems caused him to abandon the move after only a few years of use.
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.
In Dungeons & 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 Dead Gods module, there was a spell called the Last Word that was so lethal, even gods were afraid to use it. This didn't stop Orcus, however. Having been deposed and reduced to an barely alive state, used it in his camaign to restore himself to life and regain his domain in the Abyss. After killing several gods using this spell, he succeeded, but lost the ability to use it. (The gods have since taken steps to prevent anyone from using it again.)
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 using the power of the Nucleus of the Spheres threatens to drain all magic from the world, bit by bit.
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...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.
Blue Rose has Sorcery, generally banned in most kingdoms, to the point that legalizing the study of sorcery in Aldis has been greatly controversial. While arcana are generally legal in Aldis, sorcery allows an adept to directly harm, control or invade other individuals, mentally or physically, as well as create undead or summon darkfiends, and are considered crimes against all sentient beings. Trying to classify sorcery is tricky, as few arcana are clearly sorcery, and even those that are can be wielded for a period of time without any real damage to the adept if he is sufficiently resilient. Unfortunately, sooner or later, The Corruption takes hold, and the adept will either fall into the arms of Shadow, die a painful death and become transformed into a walking corpse, or try to cleanse himself even as corruption makes it harder. Many people who use sorcery are scrupulous enough to embrace the corruption that comes with it, but the temptation is always there for any arcanist, even those with the best intentions.
In the Phantasy Star series, Megid invokes this trope by name. In PS2, it's Cast from Hit Points at a painful rate. In PS3 it's only invoked in a cutscene, but when it is, it wipes out an entire city in a single casting. Note that PS3's setting is largely medieval compared to the sci-fi ambience of the rest of the series, so any spell capable of leveling a city is pretty much the equivalent of the Tsar Bomba. In PS4, the negative effects on the player are gone, potentially due to where it comes from and what the stakes are at the time, but it's not easy to access (being guarded by a SealedGood Is Not NiceIn A Can guardian who you need to outwit to get it) and the only other user of the spell is the Eldritch Abomination that threatens to wipe out the entire star system (and, ostensibly, the universe after that) if you fail in your quest.
The Chaos Dunk from Barkley, Shut Up and Jam: Gaiden is this for basketball, with the original one wiping out millions of people and getting B-Ball outlawed and is implied to kill Barkley and Balthios when Barkley uses it to kill Shadow Barkley
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.
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 (more specifically, projection using Archer's arm) 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. Which it eventually does.
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. Archer, who can create Noble Phantasm duplicates, routinely uses this technique to compensate for the fact that his duplicates are weaker than the real thing.
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. if the player is able to forgive the resident backstabbing friend Snowe throughout the course of the game (and he becomes less and less worthy of forgiveness as time goes on, so it's tough) and recruit all 108 Stars of Destiny, Leknaat appears and says that the rune's time of punishment is at an end, and the time for forgiveness has arrived. In addition to unlocking the most powerful rune attack, which greatly damages enemies and greatly heals allies, the improved Rune of Punishment no longer injuries the user for attacks. 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, a young boy has to choose between using its power or watching his village get burned to the ground by his own countrymen... It's pretty much damned if you do, damned if you don't.
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 Mega Man 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 noticeable 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.
Also doubles as Difficult, But Awesome, as enough use of Chaos Unison trains the player to be able to use it multiple times in succession without being knocked out of it without Pause Scumming, meaning the player can continue charging even if they're being pressed. Shadow Chaos, Knight Chaos, and Magnet Chaos are particularly deadly upon being mastered.
In The World Ends With You, Joshua uses his Jesus Beam attacks to get himself and Neku out of a battle with a Taboo Noise. Though not dangerous in itself, he held back this power until this moment to conceal his identity as the Composer, and used this power in the sight of a Reaper, thereby drawing suspicion to himself.
Hanekoma later proceeds to revive Minamimoto as a Taboo Noise He reveals this himself to the player in the secret reports
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...
Castlevania: Order of Ecclesia, blah blah blah, Dominus glyphs, blah blah blah, union kills you dead on the spot unless you're using it at the end of the last battle, where Albus's soul takes your place for the sacrifice.
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.
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.
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.
There's also Curse and Belly Drum, where the user sacrifices half its total HP in order to have the foe lose 1/4 HP every turn or maximize its Attack, respectively. If the user uses either of these moves when they have less than half HP left, they faint.
As of Generation V, there's a move called Final Gambit, which causes the user to deal its current HP to the enemy at the cost of fainting. It's also commonly suggested as the best move for Shedinja.
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.
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.
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.
Although the only time that the Healing Touch is actually bad is in Under the Knife, where it slaughters your ranking. Any game from Second Opinion or later have no penalties to using the Healing Touch.
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.
Zed in League of Legends is stated to have learned forbidden shadow techniques to defeat his rival Shen, and become a more powerful ninja. It mostly translates to throwing living shadows everywhere.
An antagonist example: In Golden Sun, shapeshifting into dragons seems to be a Dangerous Forbidden Technique of the Mars Clan of Prox, requiring a great deal of energy and completely wiping out the user's abilities afterward. Saturos even warns Felix in the first game that he and Menardi won't be much help after fighting Isaac & Co. as the Fusion Dragon (which they aren't, though not for the reasons Saturos had expected). When it's forced on the antagonists of The Lost Age, they are left without enough power to warm themselves against the freezing cold of the Northern Reaches, and freeze to death. And then the Wise One forces the transformation on your parents...
The Binding of Isaac allows players to pick up many upgrades and abilities at random, some of which are very, very powerful. But there are many with heavy, even potentially crippling costs.
Epicac changes one's tears into a strong explosive attack, at the cost of making them charge slowly and be able to hurt the player. In an added bit of irony, range up effects or upgrades actually make Epicac harder to use, as maxed range makes the projectile detonate far from the player. This means they'd need to hug the opposite wall just to have a hope of hitting a target in the middle of the room.
The Suicide Bomber vest allows the player to use unlimited explosions, with the downside being they also take the damage. Because, you know, they are detonating bombs strapped to their chest.
The Devil Rooms usually hold a number of power items, and while they vary from room to room they're relatively consistent. The problem comes from having to sacrifice heart containers to receive them. Not hearts, heart CONTAINERS. The player literally sacrifices their life for power. And that's not counting many of the items they get from the devil rooms have their own disadvantages.
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.
Also, the Ice-9 spell, which was purely theoretical until Red Mage used it to defeat Kary, the Fiend of Flames. No one had ever cast it up to that point because it would put everything in existence on ice. Thankfully, RM had a Bag of Holding which contained both Kary and the spell.
There are several different levels of it in the Whateley Universe. Phase has a technique (using his disruption-light level on someone) that runs the risk of corrupting their Body Image Tempalte if they are an Exemplar or Shifter, and turning thme into something grotesque; he avoids it it on moral grounds. Fey has some spells powerful enough that the energy drain will destroy entire ecosystems around her, which is way worse.
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.
Energybending also carries the danger of mentally and spiritually destroying those who attempt to use it on others if their willpower isn't strong enough. This nearly happened to Aang when he used it on Ozai.
Bloodbending, supposedly the ultimate technique of waterbending, which involves moving around the blood in a living body and making that body do whatever you want it to do. Note that it's not dangerous to the user, but it does fit the immoral aspect. In The Legend of Korra it has actually been declared illegal to use.
In W.I.T.C.H., 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. Fortunately, the one time they do this, they manage to come back- barely.
ReBoot: 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.
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 greatly tax himself reversing time by a few minutes, and eventually die by using his time/space powers to get the team through the rift separating Gigantion from the normal universe.
In Justice Leaguethe League had pretty much lost against the Brainiac/Luthor hybrid, until The Flash saves the day by running at extreme speeds (to the point where he was circling the world in mere seconds) and smacking Brainithor around by repeatedly running into him. Soon enough, he destroys all traces of Brainiac, leaving only a naked Luthor lying on the ground. 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.
A Batman The Animated Series episode has a ninja steal a scroll in Japan that teaches him the secret of a "deadly touch" technique. When Batman faces him off in the end, he tries to avoid being touched by the guy in a specific spot, which (theoretically) would cause instant death. He fails and falls down, seemingly dead. He then gets up and knocks out the bad guy. When asked if the technique was bullshit, he pulls out a metal plate from under his suit, which has been deformed by a strong force.
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 SMS Lutzow, so why not? Why not, indeed. The mist that was hiding her cleared, just long enough for the critically damaged but afloat Lutzownote At the end of the battle, Lutzow was too badly damaged to withdraw and was scuttled., along with a second German battlecruiser SMS Derfflinger, 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. The fact that British ships of World War I used a more volatile gunpowder than their German counterparts didn't help, either.
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 constitutes a war crime. Though eight nations are declared to have nuclear weapons (and one is suspected) most nuclear powers refuse to consider using nuclear weapons unless they someone else does first. Nowadays, the worry is more about the dangers of non-state actors (ie. terrorists) using them instead, which MAD is essentially powerless.
Even nukes aren't necessarily as repellant as biological warfare. At least a nuclear weapon only destroys its targeted area, however horribly; use of biological agents as weapons poses a very real risk of infecting everyone the contagion touches, be they enemy, ally, innocent bystanders, or one's own side.
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, the 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. It will eventually heal, if 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.