I Thought It Was Forbidden

"Ridcully: That would be unethical, Dean. The Dean: Why? We're the good guys, aren't we? Ponder Stibbons: Yes, but that rather hinges on doing certain things and not doing others, sir."

- Terry Pratchett

"Arthur: Mmmm. So now you've noticed how much time Cobb spends doing things he says never to do."

- Inception

There are two possible cases of this trope:

Physics (Type 1): A "Dangerous Forbidden Technique" that someone uses frequently without suffering the consequences. The Hero just uses it because he's just that awesome. Sometimes justified by the hero being able to avoid the drawbacks of the technique through experience or through unique abilities that cancel out the drawbacks. Other times, everyone seems to do it frequently, and so whatever made it forbidden is an Informed Attribute.

Ethics (Type 2): This is the case of something that's forbidden because of ethical ramifications. Ethics? Who needs those when you've got a universe to save!? No one will care if you kidnap someone from the past with your time machine. It doesn't matter that you just took that bad guy's God-given free will. Ignoring rules for "the greater good" doesn't make you anything like a villain. In better cases, this is portrayed as a Shoot the Dog, or the writers mean to let the audience decide without having the Aesop dropped on their heads. In worse cases, we get a Designated Hero.

Bad guys are often exempt from type 2 because, well, they're bad guys. Why should a Card-Carrying Villain or Complete Monster care about ethics?

Anime and Manga

 * Naruto lives off this trope - forbidden techniques are referred to as "kinjutsu" and may be either of the above types.
 * The title character knows two forbidden techniques, one which he uses all the time. To his credit, the second one starts out very dangerous for him. But, of course, he finds a way to bypass the dangerous part, because he's frickin' Naruto.
 * There's also Rock Lee, who seems to suffer less and less as he continues to use the Lotus technique.
 * All three cases do have justifications: the Mass Shadow Clone technique Naruto knows seems to have been forbidden because it'll use up a jonin's chakra in a couple of minutes. Since he has the nine-tails, he has a lot of chakra. The uber Wind-Element Rasengan got forbidden when, after using it the same way he used a normal Rasengan, it damaged Naruto's arm badly. So he learned to . The lotus technique however would likely cause less strain on the body the older and stronger you got. Case in point, Guy used six chakra gates and was only as exhausted as pre-time skip Rock Lee was using three.
 * Kakashi was forbidden to use Chidori by the Fourth Hokage  as it caused a tunnel-vision effect when used with regular eyes. Since gaining the Sharingan he has overcome this problem.
 * The Uchiha have developed so many "forbidden" techniques (all based around their Game Breaker bloodline ability) that it's a wonder they didn't all die out or go blind decades before they were massacred. Funnily enough, finds a clever way around the one that causes permanent blindness in exchange for the ability to warp reality:.
 * Filler character Guren uses multiple supposedly-life-sacrificing techniques consecutively, while bleeding heavily, without hindering her fighting ability much.
 * Dragonball Z - the Kaio Ken (literally "World King Fist") technique briefly amplifies every aspect of the user's being - from physical strength to sense of taste. Goku is warned that his body may not be able to handle such a surge of power. While he does feel pretty bad the first few times he uses it, later on he suffers no such effects. By the time he fights Frieza he can stack the technique up to 20x its normal power and doesn't need to even call it anymore. King Kai has to explain to other characters observing the battle that no, he's not holding back he's been using it the whole time.
 * Initially averted in Slayers. Lina's improvised ultimate spell, the Giga Slave, is at least noted as a way to deal with every big bad. However, because of its side effects, great effort is put into not using it because It ends up being used only thrice, once before it's full ramifications are known (And really wouldn't be much worse than not using it had they known) with a number of additional measures backing it up, and once, Lina botching it only to be saved by a literal Deus Ex Machina she may have had an inkling of in advance. However,
 * Projection in Fate/stay night for Shirou; he keeps doing it and it says that it burns out his nerves and magic circuits and whatnot and yet it never permanently injures or kills him — Shirou himself postulates in "Unlimited Blade Works" that for all its Forbiddenness, the strain of doing it will never be enough to permanently harm him..
 * In One Piece, the main character Luffy has a technique called Gear Second that makes his blood pump faster, increasing his speed and power. In the first arc this appeared in, it was stated that using the technique wore him out, while also decreasing his lifespan. But by the Impel Down and Marineford arc, he's constantly using the technique, even when he doesn't really need to, with seemingly no side effects at all.
 * During the Skypeia arc, one of the characters Wiper had a special weapon that could put out a lot of force, but which also backfired on the user. The weapon could potentially kill a user with one use. Throughout the saga, Wiper uses it three times, and though it injures him, it never takes him long to get back on his feet.
 * Using the Ginzouishou to its full power is said to be deadly in Sailor Moon, and it does kill Queen Serenity. On the other hand, Usagi uses it about once a season and, despite a few close calls, she always survives none the worse for wear.
 * Singing the Ultimate Song in Senki Zesshou Symphogear is supposed to kill the singer, or at least put her out of commission. By the end of the series, the protagonist trio use it twice in the same fight.

Literature

 * In Robert Jordan's Wheel of Time series there exists a nasty spell called Balefire, which basically erases its target from existence. Retroactively, with Ripple-Effect-Proof Memory. The more force you put into the spell, the farther back in time it goes. The heavy strain this spell put on the very fabric of existence was enough to get both sides—light and dark—to silently agree to stop using it long ago. For all its dangers it can also be extremely useful. Several characters are saved (or revived) from death when their attacker is hit by this, it ignores defenses, and it's a guaranteed way to . In small doses there aren't any apparent ill effects. It's only with massive blasts that the world seems to momentarily sway to the characters nearby.
 * Max Impeller compensator rate in the Honor Harrington series is treated like this. Word of God is that it's a historical artifact from a previous era when going to the maximum rate really wasn't that safe and risked catastrophic failure of the compensator, which at 500G would turn you to paste. Of course we've never seen a ship have random failure in the entire series even at max rate as modern designs don't need nearly the same margin for safety.

Live-Action TV

 * Star Trek: While not an attack, the Vulcan mind meld is originally set up this way. It can cause irreparable harm to the minds of both people. Nevertheless, it becomes part of Spock's standard tricks throughout the rest of the series, and becomes mundane even later. Even in the prequel it is used by someone who doesn't even know how without even a mention of harmful effects.
 * In the first-season episode "Arena", Kirk's order to exceed the Enterprise's maximum safe cruising speed is heavily Played for Drama. Similar and higher speeds are used in later episodes with much less concern for the danger.
 * In Kamen Rider OOO, each third of the suit has different modes (instead of the entire suit like most riders). The mechanism is too complicated to go into, but using three of a kind is very powerful and very likely to kill you or drive you mad. And usually, Eiji is warned not to, does it anyway, and may faint afterward. He might act more feral for the first few seconds of its use, too. However, by this point, two episodes don't pass without the use of it, and it seems to be no big deal. He later gains another combo which is even more dangerous than the normal ones, but apart from sending him into an Unstoppable Rage the first few times it follows the same pattern.
 * Especially for Kamen Rider, where such things are often as bad as they're said to be (you really do die if as a normal, unaltered human you use the Dark Kiva or Kaixa powers.)

Video Games

 * In videogames you sometimes get to use the dangerous forbidden stuff without ramifications because they don't want to limit the player's freedom of choice.
 * Jedi Academy: Your teachers get worried if you learn many Dark Side powers, but whether you fall to the dark side in the end is entirely up to player choice.
 * World of Warcraft: The warlock class. Summoning demonic servants and tormenting your enemies with dark curses places your soul in no peril if you're a player character.
 * Players in-game don't know much one way or another about the state of their soul, so maybe warlocks are damned, along with everyone who completes the torture quests; we never find out. And warlocks do pay a price for their power, or at least they used to before the Doomguard summoning spell was changed, and there's always Life Tap.
 * Mentioned below in Type 2 is the Final Fantasy XI Blue Mage, but what is discussed even less is the implication that pursuing this path will also turn the person into a Mind Flayer upon their death, or if they grow too strong.
 * MAR has it's titular character Ginta with his ultimate technique, third ÄRM, Gargoyle which turns Babo into an extremely powerful beast, complete with lazer breath. He is warned early on (in a flashback) not to use it heavily, as it is taxing on him. First time he is seen using it, it drains him. Every onther time after that, he's flashing it out so many times it's a wonder why he doesn't just start his fights with that and be done with it.

Movies
"Peter: 'Scuse me Egon, you said crossing the streams was bad..."
 * Lampshaded in Ghostbusters when Egon suggests crossing the streams.

Anime and Manga

 * In Fullmetal Alchemist, human transmutation is forbidden due to the ethical ramifications.
 * In the 2003 anime version,
 * Despite mass-based weaponry being illegal in the Nanoha-verse, in Magical Record Lyrical Nanoha Force, the main characters turn to devices that have mass-based attacks in order to combat the Huckebeins' Anti-Magic abilities.

Film

 * Kind of inversion in Mr. and Mrs. Smith: after witnessing how dangerous the bazooka is, Mr Smith tells it should be forbidden. It is. As is his job.

Literature

 * In His Dark Materials: touching someone else's dæmon is given this treatment (sort-of), though admittedly mostly by the villains. In book one, it's treated as a case of "Never, ever, ever. Not even in battle, when trying to kill another soldier, will you touch his dæmon." By the time of book three, the standard seems to have been significantly relaxed - then there's the odd times when a dæmon will initiate contact with another human. In the companion book Once Upon a Time In The North, the Complete Monster of a villain talks about the number of times he's manhandled other men's dæmons to torture them. However, one could argue that the standards for touching someone else's dæmon have always been at the levels that are shown in book 3, but Lyra, being a child, was just unaware of them.
 * On the other hand, in The Amber Spyglass when
 * Given that many some of the incidents in which people touch others' demons have sexual overtones (When a mook touches Lyra's daemon, the phrasing used could have applied to him molesting her just as easily, ,) this could have some interesting connotations.
 * In the Discworld novel Wyrd Sisters, Granny Weatherwax mentions that they can't restore the rightful heir to the throne of Lancre by using magic. Then they have to use quite a lot of magic to get the rightful heir back to Lancre so he can get his own damn throne back (whether he wants it or not). As Nanny Ogg says, "When you break rules, break 'em good and hard."
 * In Witches Abroad, when Magrat again complains about Granny Weatherwax meddling in things she says witches aren't meant to meddle in, Nanny explains it's all down to the difference between the general and the specific. "When Esme uses words like 'everyone' and 'no-one', she ain't including herself." Mind you, earlier in the same book Magrat offered the highly questionable rationalisation "It can't be bad if we're doing it. We're the good ones."
 * There's a subplot in Survivor's Quest where Luke, three years married to Mara Jade and happy about it, is aware of the Jedi's stance against attachment, including love and marriage. One of the reasons he's eager to visit the ruins of the Jedi-headed Outbound Flight is his hope of finding intact, unaltered Jedi records which might, just possibly, have something in them about Jedi who disagreed with this stance.
 * In the final Harry Potter book, Harry and his friends start firing off "unforgivable" curses pretty freely.
 * Also prevalent in the movies where villains are frequently vaporised by magic or magically torn to pieces by heroes. But it's all fine so long as nobody uses the "instant painless death" curse.
 * The implication in the books is that while anyone can tear someone limb from limb, the killing curse is inherantly evil, as it is fueled by the desire to end a life.
 * In Tigana, Alessan  in order to save the world (or actually, just one country).

Live Action TV

 * Star Trek's Prime Directive. It tends to only be mentioned when the captain is about to break "bend" it for the greater good.
 * There were, however, a number of episodes (none in the Original Series...) that subverted this, with the captain indeed trying to enforce the Prime Directive in morally... troubling circumstances. The implications were often very disturbing. (Stop a deadly illness that we've got a cure for from wiping out an entire civilization? But that would be playing god! Save a primitive society of millions from a supernova? But we don't want to interfere! Etc.)
 * More disturbingly, this also happened in Enterprise when the Prime Directive didn't even exist yet! No rule was forcing him to act this way at all... it came across more like he just didn't want to help anybody and helped derive the Prime Directive as an excuse!
 * The ending was changed from actually saving the species to what it became due to Executive Meddling.

Tabletop Games

 * There are several rules to Time Travel in Continuum, and one very specific rule is to avoid Frag and fix any encountered. Only a Narcissist would be so vile as to intentionally risk the lives and futures of the universe for their own selfish ends. And then most time combat is about fragging the opponent as fast and as hard as possible, often to the point where the enemy is little more than semi-aware goo swaying through the true universe they are now barely able to touch. Unlike most cases, this is called out, although the danger presented by and difficultly in defeating normally a Narcissist makes it a Necessarily Evil.

Video Games
"Tidus: You can stuff your taboos!"
 * This can be easily applied to players in Video Games, as while the story has ethical ramifications, it's not actually real, but most gamers couldn't care less in their pursuit of shinies. Case in point: In Final Fantasy XI, becoming a Blue Mage is seen as power-hungry and amoral (you pretty much sell your soul for it). Of course, nobody cares, because BLU is one of the most asked-for jobs andIwantmyBlueMagedammit!
 * It's not really a Deal with the Devil, considering the Empire drugs and abducts prospective Blue Mages and forces the procedure on them against their will. In-world lore still portrays it as amoral and self-destroying, however.
 * In Final Fantasy X, Wakka informs Tidus that it's taboo to enter the Chamber of the Fayth unless you're a summoner, but he's ignored every time. Ultimately, it turns out the taboos were put there in the first place for very bad reasons...


 * A more practical reason might be simply not wanting people going in unnecessarily, since the chambers tend to be death traps and they want to minimize casualties. You can get roasted alive in Kilika, electrocuted in Djose, frozen and left for dead in Macalania, and fall into a for-all-points-and-purposes bottomless pit in Bevelle. Besaid, though, doesn't have a justification for this other than the whole "Yevon says so" line.
 * In Dragon Age, people talk a whole lot about how evil blood magic is. You can nonetheless walk around with a party of 3 blood mages, and nobody will bother you about it. In fact, nobody will even seem to notice, no matter how blatant you are about your use of forbidden and dangerous magic, not even the Knight Templars who are supposed to kill blood mages (or your party members who belong to said Knight Templar organization). It's also explicitly stated in the Warden's Keep DLC adventure that the Grey Wardens don't forbid blood magic, which may be part of their poor relationship with the Chantry.
 * In the sequel, you can roam the streets at night exploding bandits with your awesome power. Not once will you ever be accosted for being a mage roaming freely through the streets. Which is forbidden. This is Handwaved at a couple points, at first by your contacts and later by your social standing.
 * You can chide Merill for using blood magic when you first meet, and people will keep mentioning how she's dabbling in things she shouldn't be, even though the player might opt not to use any of her blood-related magic after that first scene. And at the same time, you can already have blood magic yourself, and thus start any battle by stabbing yourself brutally in the stomach and then keep on rolling by using people's own (still internal) blood against them in the most gruesome of fashions.

Western Animation

 * In Time Squad, Larry 3000 points out that kidnapping people from the past is forbidden (yes, he used that exact word), and they do it anyway. To be fair, kidnapping someone made them actually able to do their job properly.