Nice Job Breaking It, Hero/Tabletop Games

Examples of in  include:

Board Games

 * In Warhammer 40,000, Magnus The Red used sorcery to become aware of Horus's Face Heel Turn and desperately broke his oath never to use psychic powers again to immediately inform his father the Emperor that Horus was planning a bloody revolt against him. The Emperor, refusing to believe that his favorite son could turn against him and possibly influenced by the widespread Fantastic Racism against mutants like Magnus, instead ordered Leman Russ, who was already looking for an excuse to take out his personal grudges against Magnus, to apprehend him and bring him to Terra. The Thousand Sons legion might not have defected and brought the Chaos Marine forces to exactly 50% of the existing legions at the time had Russ not gone in guns blazing and tried to execute Magnus on the spot. Instead, Tzeench got his own personal legion of chaos marines and the most powerful psyker since the Emperor joined Chaos. Nice job, guys.
 * To be fair, his standing orders from the Emperor were to go to Prospero (the Thousand Sons' home planet) and arrest Magnus, preferably to be done without shooting. However, Horus appeared and told Leman Russ that his orders were to kill Magnus, which is the reason why the Space Wolves/Thousand Sons grudge started (and continues as of the 41st Millennium).
 * The Horus Heresy series adds an extra layer to this- the Big E didn't send Russ just for Magnus breaking his rules, but because Magnus caused the biggest Nice Job Breaking It, Hero moment of the entire series. In trying to warn his father about Horus, Magnus psychically travels through the warp to Terra, but finds it blocked by extremely powerful wards. The rage from his apparent failure leaves an opening to Chaos to offer him enough power to break through, which he does so with haste. End result- Magnus gets his message through, but finds that not only did he destroy all the wards protecting Terra, but the psychic back-blast also fried the Golden Throne, which was supposed to be used to contain Chaos and/or access the Eldar webway. He then suffers My God, What Have I Done? and returns to Prospero to wait for Russ to arrive and kill him.
 * The selection of Leman Russ probably indicates the Emperor actually had at least some consent for the Space Wolves to attack. The Space Wolves at that time were the only group of loyalist Space Marines that did not find the concept of fighting other Space Marines to be completely incomprehensible. Very early in the Horus Heresy series, an entire Luna Wolf squad was killed by a single Space Marine that was infected by a demon because not a single Space Marine in their right mind could bring themselves to hire on one of their own, even to preserve their own lives, the lives of their squadmates, or even to ensure they were capable of completing their mission objectives. He could have sent several other primarchs, but he chose to send the Primarch with a grudge, a violent reputation, and a complete willingness to fight other Space Marines. The only other explanation is Plot Induced Stupidity.
 * It is worth noting, that at the time, the Space Wolves were in fact trained to destroy other Space Marine legions.
 * Another chance to break it, hero in WH40K is the Lords of Change. They receive visions of the immediate future from their master Tzeentch, making them virtually impossible to kill. It is possible to kill them--but only if they receive false visions of the future from Tzeentch. So by killing a Lord of Change, you're furthering Tzeentch's schemes.
 * Though, to be honest, as Tzeentch's schemes have no actual goal besides "make things change", you can't not further those schemes. Every action you take, every thing you do, even whether or not you were born, is all just part of Tzeentch's plan of seeing how the universe will move if he does this or that.
 * Pretty much every major crisis in the Eisenhorn series was caused as a direct result of Eisenhorn's actions, which led to a fairly minor situation getting progressively worse and worse.
 * The Emperor winning the Horus Heresy is an example of the trope, according to the novels of the same name. It was prophesized that had he lost, humanity would have died out in a couple generations taking Chaos with it and freeing the rest of the galaxy from it. This is the entire reason Alpha Legion decided to join Horus.
 * The Emperor did this on more than one occasion, according to the backlore for the game setting. In fact, the whole Horus Heresy could, in part, be considered the combined payment for many of the Jerkass things he did to his "sons", the Primarches.
 * For example, when Lorgar of the Word Bearers encountered the Emperor, he transferred his deep religious faith into deifying the Emperor, and indoctrinated his Legion of Space Marines to believe the same way. As a result, when they pacified and claimed a world, they spent an extended period of time erecting temples to the Emperor and establishing their religion as the planet's new sole form of faith before moving on to the next one. The Emperor said not a word about this for centuries, then suddenly chastised the whole Legion for both "dawdling" and for believing in him as a god in the first place. The Horus Heresy novels explain that this chastisement took the form of going to a planet the Word Bearers had claimed over a century ago, which they regarded as the jewel of their achievements for the willingness and extent they had adopted the Imperial Cult, and then razing it to the ground. Then the whole Legion, including Lorgar, were summoned to the ashes of their world and telepathicly forced to kneel and listen to the Emperor chew them out -- and in front of others, for added humiliation. Lorgar goes on to fall into worshipping the Chaos Gods and is instrumental in the corruption of the rest of the soon-to-be-Traitor Legions.

Card Games

 * Urza from Magic the Gathering pretty much lives for this trope, between the sylex blast, the soul bombs and the Tolarion Academy he probably killed as many people as Yawgmoth did in the invasion.
 * Urza's, and by extension, Mishra's, ultimate example of this page's topic was actually their shattering of the Powerstone that held closed the original portal to Phyrexia, allowing the entire next decade (or, in-world, 2,000 years) of storylines to happen, and ultimately tore down reality enough to cause a Continuity Reboot inside the continuity.
 * Jace Beleren. After getting locked in the Eye of Ugin with Chandra and (a super-sized, very draconic) Sarkhan Vol, he realizes that their spells are useless...then he realizes the "sheer flame" the scroll that led him (and her) there speaks of would work to take down Sarkhan and save both their hides. It does....it also unlocks the Eye, unleashing Ulamog, Kozilek, and Emrakul, aka the Eldrazi, destroyers of planes of existence.
 * Goes double for Nissa, who decided to destroy the Eye in the hope that this would make the Titans leave. It didn't.
 * Similarly, Glissa Sunseeker of Mirrodin block set out on a quest that ended with the overthrow of the mad golem Memnarch, whose "leveler" crushing robots killed her family. Then it turned out that despite his arbitrary and despotic habits, he was the only thing standing between Mirrodin and Phyrexian corruption. Now, a few in-game centuries later, we have Scars of Mirrodin block, with the final set confirmed as New Phyrexia...and Glissa herself has been corrupted by the glistening Phyrexian oil. Yeah, It Got Worse.
 * Karn's heart is a Phyrexian power stone. It's almost all that remained of Phyrexia after their invasion of Dominaria. The new Phyrexians consider him their father, and it's entirely possible every plane he ever walked on has the Phyrexian corruption.

Tabletop RPG

 * In a Dungeons & Dragons adventure featured in Dungeon magazine, an evil giant living in a flying castle waged a terrible campaign of vengeance upon human towns and villages, murdering scores of innocents in the process. If the heroes killed him instead of making some sort of agreement with him, however, castle dissolved... and released an unspeakably powerful god-spawned monstrosity from its centuries-old prison. The monstrosity would then begin methodically and efficiently killing everything in the area, followed by everything else on the planet. Whoops.
 * Potential for this trope is built right into the Ravenloft setting, where eliminating a domain's darklord can have three possible effects: A) another evil being is elevated to darklord status, gaining immense power; B) the domain is split among neighboring darklords, potentially kicking off invasions and so forth; or C) the domain -- and everyone living there -- literally disappears.
 * Years ago, a French tabletop RPG magazine had released a two seasons campaign for a generic Dystopia Twenty Minutes Into the Future setting. Season one had the players going against a Nightmare Fuel Psycho for Hire known as the Butcher, who was trying to initiate the biblical apocalypse. They were helped in their quest by a mysterious cube, which, between fast-paced action sequences in the present, allowed them to time travel via mind-transfer to Set Right What Once Went Wrong. Then in season 2, it is revealed that The Butcher was really acting this way to prevent a Bad Future to occure (effectively willing to sacrifice millions of people to save billions later), and the players have been manipulated into opposing him all along, the cube actually being a gift from some Cosmic Horror, with which they really were setting worse what once went wrong -- and now of course they have to clean their mess, by time traveling again, this time with a cube given by The Butcher, all while fighting Big Bad 2 The Plague, a Nightmare Fuel Complete Monster sweating Body Horror and borderline Eldritch Abomination, who is actually one of the secondary antagonists of season 1, Left for Dead by the players and "reconstructed" later -- oh, the Irony! (for extra irony, the second cube, with which the characters are supposed to fix what they spoiled, is actually much less user-friendly than the first one).
 * In Scion, the titans that the titular heroes have to fight against are innately tied to nature itself, and their deaths have dramatic effects on the planet. For instance, when Odin killed the titan Ymir, it ended the ice age. All well and good, except that this is what caused the Great Flood. One can only imagine what would happen if Gaia or Kamimusuhi, titans of birth, were to be killed. Fortunately, there are cans to shove them in.
 * Among the many, many potential elements of Paranoia Fuel in the New World of Darkness is the potential for members of the Vigil to do this, due to the fact that most people are either genuinely ignorant of the subtle shades of darkness of the supernatural, too closeminded to accept that things aren't black and white, or both. That pack of savage shapechangers? They're the descendents of a long line of half-mortal half-spirits whose purpose is to keep alien totemic spirits from ripping through the fabric of reality and turning humans into puppets and food. Those crazed self-proclaimed mystics? They're actual wizards trying to restore a golden age of humanity, as well as fight off invasions from a kind of 'anti-reality'. The vampires running a trendy nightclub and secretly bleeding the human clientele? Now the civilised vampires are gonna start being a lot more brutal in their feeding habits as they struggle to find their own prey... to say nothing of the band of sociopathic-even-by-their-standards vampires who are going to take advantage of that opening to start butchering humans for the hell of it.
 * Old World of Darkness has its share of this:
 * The original Hunters aren't any more likely than their Spiritual Successors to have a clue what's going on or which horrific monsters need to be destroyed and which are actually the last thing standing between the local town and a horrible, horrible death. The werewolves and other shapeshifters, for example, are fighting and dying to protect their human charges from literal incarnations of hate, torture, insanity, rape, and other pleasant things.
 * Then there's Werewolf: The Apocalypse itself, where it was made clear that the main reason the Garou were doing so badly against the Wyrm was because they slew first, asked questions later. Their millennia-long righteous campaign of purity resulted in the extinction of three races of Changing Breeds (and one of their own tribes), the surviving Changing Breeds severely distrusting them and each other, and a general species-wide feeling of, "Great, now what?"
 * Archduke Dulinor in Traveller was a Well-Intentioned Extremist who killed the Emperor in the hope of invoking a precedent that the Emperor's assassin could assume the throne. This backfired when the other nobles refused to accept his claim, the Imperium collapsed in an incredibly bloody Civil War and history remembered him as a Complete Monster responsible for the deaths of billions. Ooops.
 * Happened to several sets of would-be world-savers in Exalted. Perhaps the best example would be the Usurpation, a noble if excessive action to protect the world from its insane custodians, the Solars. They succeeded, but 13 ghosts from the slain Solars became the Deathlords, with enough power to beat up even the strongest Sidereals with rolled-up newspapers; many of the Lunars of the time went insane, with Raksi, Ma-ha-suchi, and Leviathan being the worst affected; the Dragon-Blooded Shogunate proved to be nowhere near as good at handling the world as the Solar Deliberative despite the reduced risk of its members going insane; and then the Deathlords unleashed a plague that killed 90% of the world's population. Whether or not this is an improvement over what would have happened otherwise is hotly debated to this day, both in and out of setting.
 * The situation becomes more complex when its revealed that if a minority faction of Sidereals opposed to the Usurpation hadn't gone to the Underworld to question ghosts about the Usurpation as deep background research, because they were trying to get enough inside info to find the lost Solar sparks and undo its effects, they wouldn't have stimulated those ghosts' memories of who they were or how they died and thus the Deathlords would never have come to be.
 * Many Planescape adventures do that to the players. For example, Fires of Dis force them to