Brianna from the Gone series, who during Fear, attempts to rescue Diana and her baby... only to have the Gaiaphage possess it, by way of Penny's visions.
Nakajima Akemi of Digital Devil Story could be a poster boy for this trope.. Sure, he successfully develops the Demon Summoning Program. Unfortunately, he failed to build in proper safeguards that would impose a contract on a summoned demon. The great demon lord he summons turns on him very quickly, resulting in the deaths of at least half his classmates, as well as many innocent bystanders. To top it all off, after Nakajima defeats Loki, the demon he originally summoned, his teacher, Ohara, ends up summoning Set, the most powerful evil god of ancient Egypt. As icing on the breaking cake, many demons have sensed the pathway Nakajima opened to their world, kindling their ambition to conquer the human world. More than just a handful of those demons want to ally themselves with Nakajima.
In the Anita Blake novel Blue Moon the entire plot's problems, involving hostile vampires, necromancers and werewolves stem from Anita SHRIEKING ABUSE at another city's paranoid Master over the phone, and then stomping into his territory with a bunch of very powerful vampires and weres.
In the regular series, Elfangor tends to come across as noble and wonderful, a cross between the mentor and The Minnesota Fats, willing to break his people's laws, yes, but only to serve the greater good. In The Andalite Chronicles, however, we see that Visser Three's capture of Alloran is a direct result of Elfangor refusing to kill a mass amount of unhosted Yeerks. This capture also led to Visser Three's eventual promotion. While he originally gained prominence by being the Yeerks' authority on Andalites, his brutish ways and lack of subtlety wouldn't have let him progress much further. Thanks to Elfangor, who opened the door for him to do what no other Yeerk could, Visser One's agenda is the only thing keeping Visser Three from declaring an all-out space war on Earth!
Seerow being responsible for the Yeerks becoming a danger in the first place. He provides scientific technology to the Yeerks, which they use to go on galactic conquest. He becomes the Rule-Breaker Rule-Namer for the "Law of Seerow's Kindness", an Alien Non-Interference Clause that prevents technologically advanced species (the Andalites) from sharing technology with anyone else. As we see more of the Broken Pedestal nature of the Andalite command, it becomes apparent that some of this law was just pride.
Also a major one in the backstory of the Ellimist. Before he became what amounts to Q sanstwisted sense of humor, he attempted to stop an interplanetary war between two non-FTL-capable civilizations by towing a bunch of asteroids between their planets and then blowing them up, creating "an impenetrable orbital minefield" with the debris. Unfortunately, this just gave one side the idea to plant nuclear mines in the path of the other side's planet, glassing it, and being suddenly without an enemy for the first time in centuries the victors' civilization basically self-destructed.
In the first book Harry rushes to save the Philosopher's Stone from Voldemort. Turns out the Stone was hidden so that only a "pure of heart" (basically) could retrieve it, which Harry does, and the Stone nearly falls into Voldemort's hands.
In the second both Harry and Ginny write in Tom Riddle's diary, unknowingly giving him valuable intelligence and almost allowing him to become corporeal.
In the third, Harry spares Wormtail by proxy, only to have him escape and help ensure Voldemort's return, but hey, at least Buckbeak and Sirius live!
Goblet of Fire: Harry comes up with the idea of both he and Cedric grabbing the secretly-transportational trophy simultaneously; once they arrive at its destination, Cedric is Kadavra'd. Whoops. Of course, if just Cedric had grabbed it, things might have turned out even worse since there wouldn't have been anyone to alert the wizarding world (or at least Dumbledore) of Voldemort's return.
Worst of all in Order of the Phoenix, where Harry's desperate attempt to rescue Sirius winds up drawing Sirius to his death, instead. The upside — Voldemort being exposed as alive — pales in comparison to the loss; ironically, since the entire book was about Harry's frustration with not being believed and attempts to make people believe him, only for Harry to not give a damn anymore when he was shown to be right.
And, to a lesser extent, in the same book, when they manage to break every damn prophecy and all the Time Turners in the Ministry of Magic.
At one point in Half Blood Prince, we see inside the memories of a Ministry official who was responsible for the arrest of a father and son who abused the daughter of the family, enabling her to go after the Muggle love of her life. All very well, right? Well, there's just one teensy-weensy problem said daughter happens to go on to become the mother of the most evil and maniacal wizard this century after using Love Potion to turn her crush into her Sex Slave.
In the same book we learn R.A.B. stole a Plot Coupon and replaced it with a fake and a taunting note, which forces the main characters to find it again later because R.A.B. didn't manage to destroy it. Especially annoying because of the circumstances of pointlessly gaining the fake.
Subverted in the Deathly Hallows. Harry realizes he must let Voldemort kill him. As this was all part of the plan, and Harry suffered no more than a bruise, it all worked out.
In a more subtle example, the majority of the population tends to call the Big Bad by impersonal aliases, like You-Know-Who or The Dark Lord, while Dumbledore calls him by the name and encourages others to follow suit. This backfires in the last book, when Voldemort charms his own name, so that whoever utters it will be stripped of all wards (except the Fidelius Charm) and exposed to his minions. Which leads us to Harry blurting out Voldemort's name despite just being told not to resulting in each and every magical defence around their tent all crashing down at the exact same time. You could construct a very decent argument here that this makes Harry directly responsible for Dobby's death.
Also in the last book, not that he was much of a hero in the first place, Mundungus Fletcher's cowardly and stupid act of chickening out by disapparating inadvertently got Mad Eye Moody killed, arguably Mundungus' worst mistake. He then followed up with another blunder—stealing Salazar Slytherin's locket as if it was something for the market and end up being bribed into giving it to Dolores Umbridge, who he was possibly unaware had become loyal to Voldemort's new regime. After those huge blunders, Harry, Ron, Hermoine and everyone else would refuse to work with him again.
Back in his school years Hagrid releases his pet giant spider into the Forbidden forest to preserve it from being killed and even finds it a mate. Fast forward and, what a shock, they spawn a swarm of vicious monsters, whom the Death Eaters later use against the school in the Final Battle.
In the SF novel Legacy of Heorot, human colonists on an alien planet manage to eliminate the "grendels", a native, komodo-dragon like species, that threatened their existence. They find out too late that the grendels were the mature (female) form of these tasty (male) "fish" that are just about everywhere. (Think frogs and tadpoles. Only the frogs are a lot bigger. And have big, pointy teeth. And decide that people taste good.) The mature grendels kept the population down through cannibalism. Now that there are no more mature grendels all of the immature grendels start to grow up and undergo metamorphosis, and they're hungry. Even worse, all of them can move almost faster than the eye can see. Guess what they decide to snack on Nice job breaking the food chain, heroes.
This is more or less how the Sword of Truth series moves from one book to another: Resolving the conflict of one book leads directly to the problems in the next, at least in the beginning.
Happens not once but twice in the Eoin Colfer novel The Supernaturalist. The eponymous group are hunting ghost-like creatures that they can see congregating around injured and dying people. They want to destroy them to prevent them from sucking away people's life force. Too bad they do nothing of the kind. They're benevolent creatures that feed on pain, easing people's suffering. And the kids' killing them off causes them to crop up in larger and larger numbers, so that many of them begin starving to death.
In The Edge Chronicles:Vox, in what should be the hero's Crowning Moment of Awesome, he fights to stop a minion triggering an alchemical bomb by pouring water into it. During the fight it's stressed that the bomb is so unstable that one drop of water might do. At last he beats the minion and throws his jug of water far away... sags exhausted over the bomb... and a drop of sweat falls from his face into the bomb.
Done several times in Brandon Sanderson's Mistborn trilogy.
First, at the end of the first book when Vin kills the Lord Ruler he warns her that she has no idea what he does for mankind and that she has doomed the world. In the second book, the protagonists discover that the mists that have covered the world for the past thousand years are growing thicker and destroying crops. In addition, the mists are killing a small fraction of the people that go out in them.
To stop this, Vin finds the Well of Ascension, which is rumored to have the power to stop the mists. When she finds the well, she releases the power in it as described in the prophecies. Rather than saving the world, this releases the Sealed Evil in a Can. Oh, and the mists are still getting thicker, but that's the least of their problems from this point on. It turns out that the mists were not created by the sealed evil but his now-defunct opponent. He just made them stronger in order to distract the heroes from his true goals. He also perverted the prophecies to play out his plan. Luckily, it turns out his opponent had anticipated him breaking free and planned for it, but it was still a close call.
Also, in the backstory, the Lord Ruler, when he held the power at the Well, tried to burn off the mists by moving the planet closer to the sun to make it hotter. He moved it too far and made the world too hot, so he tried to push it back and made it too cold, then he tried to put it back into its proper orbit and made it too hot again. He finally gave up on that tactic and created the Ashmounts to spew ash into the air to cool the planet, which would have killed all the plants, so he altered the plants to survive, which then required him to alter the people and animals to eat the altered plants and not choke to death on the ash. The end result of this was an ash-covered Mordor landscape where the sun was red and the plants were brown that was the setting for the trilogy.
And in the third book, when Vin tries to undo that, she almost destroys the planet.
This happens in Warbreaker as well. Vivenna is deceived into furthering the plans of the villains. Denth tricks her into helping to start a war between Hallandren and Idris. After discovering this, she spends the rest of the book trying to make up for her mistakes by helping Vasher prevent the war.
And it happens in Elantris, too, when Sarene's curiosity leads to King Iadon being deposed. Sanderson really likes this trope.
In the backstory of The Wheel of Time series, Lews Therin comes up with a badass plan to re-imprison the Dark One. Unfortunately, something goes terribly wrong, and from then on all male channelers are doomed to go mad, wreak havoc with their immense powers, and then die horribly. The world gets Broken. This was also his last resort. His original plan involved both men and women going to fight and seal the Dark One, but the women refused in favor of another plan. One which fell further and further out of reach as the war raged on. The women weren't changing their minds, even after several years, so he had no choice but to go with men only.
It has been argued, however, that Lews Therin being forced to go with only men actually mitigated the disaster, in that had both men and women participated in the resealing, the Dark One would have been able to taint both halves of the One Power, multiplying the resultant destruction and leaving no Aes Sedai (One-Power-users) at all to fight him in the coming age.
In the final volume of Tad William's Memory, Sorrow and Thorn trilogy it is revealed that the swords the hero had been trying to bring together to defeat the Big Bad were in fact the only things which would release the Big Bad from his prison. Whoops.
In the Star WarsExpanded UniverseNew Jedi Order series, it is implied (or outright stated, whichever) that Palpatine formed The Empire and ordered the construction of weapons such as the Death Star to prepare for the invasion of the Yuuzhan Vong, a species whose sole purpose is taking evil to a level that Palpatine could only dream of, making the original movies a Nice Job Breaking It Trilogy. However, it has also been commented that since The Empire lost to the Rebels, they might not have done as well against the Vong as they would have liked to think. Also, Palpatine wanted to rule the galaxy before he knew about them; his motives were not so much altruistic as "They're going to take my stuff!"
Not only that but the death of Palpatine created a vacuum of power. It caused a fracture of the Empire into multiple smaller dominions led by psychotic warlords, who, to solidify their positions, hold even worse weapons, like the Sun Crusher. It caused Admirals (and the last Grand Admiral) to start ploys to resurrect the Empire, raising the specter of a new round of Clone Wars, a working prototype Death Star, and various Super Star Destroyers. It allowed for a cloned Emperor, if possible even more insane than the original, to unleash Devastators on Coruscant. It allowed for criminal organizations to flourish and get into the superweapon race themselves, like the Hutt's Darksaber. It opened the way for genocidal races to use left-over Imperial ships. The New Republic isn't even stable and threatens to collapse many times.
This didn't even take a long time to occur. The Truce at Bakura, which starts right when Return of the Jedi ends, is a perfect example. The Rebels intercept a message for the Imperial fleet at Endor, which had just retreated, about an attack at Bakura. They decide to send reinforcements as a PR boost, which consisted of one antique carrier and a small group of fighters, many of which are on the verge of breaking down. (They do note that it's all they really had to spare: most of their surviving capital ships were severely damaged during the battle, and they were worried somebody in the Imperial fleet might get control of the situation and counterattack.) This leads to Bakura being a bloody, drawn-out battle. One Imperial Star Destroyer would have ended the battle almost immediately because it would have immediately outclassed the entire alien fleet. On top of that, the alien fleet only attacked because a Force-sensitive boy on their side felt Palpatine die through the force.
Though considering that The Empire was supposed to have a million star systems and untold resources, and yet it fractured so easily and couldn't even send a Star Destroyer to Bakura even though they only lost about 30 at the Battle of Endor when they should have had thousands left, and the reason there were psychotic warlords vying for control was because the Empire put them in positions of authority in the first place, maybe the idea that it staying for the long run was a good thing was misplaced.
And then, there is the hero's unleashing of Abeloth, which took over fifty years of in-universe heroes inadvertently doing the one thing that could damage the can holding her. First Anakin killed the anthropomorphic personifications responsible for remaking the can every few thousand years as it breaks down, then the heroes later destroy the superweapon keeping the can locked, then they use the cluster of black holes she's locked in as a fortress. Most impressively, despite all of this hero-breaking, the can still managed to hold an extra decade.
In The War of the Ancients, Malfurion Stormrage turned Xavius into a tree. Fast foward to Literature/Stormrage, where Malfurion is being held captive by said tree, which has become the Nightmare Lord thanks to an Old God and is gaining enough power to destroy the world.
Nearly happens again in Stormrage. Malfurion manipulated events so that a certain axe would be brought in that he could use to destroy the Nightmare Lord. The Nightmare Lord almost corrupted the axe so he could use it.
In the first book of Monster Hunter International, Owen uses an artifact to bring himself and his companions Back from the Dead by reversing time. Unfortunately, he also caused a mass panic all over the world and woke up the Bigger Bad. (Though given that he and three of the people he brought back are the only ones who can stop said Bigger Bad and he might have woken up again later, this might actually be good.)
In the First The Chronicles of Thomas Covenant Covenant's breaking the Staff of Law in self-defense has only good consequences. Then in the Second Chronicles the act turns out to have enabled continuing and rather imaginative evil and suffering on a massive scale. Cue a lengthy quest to repair the damage.
In the third book of Alastair Reynolds' Revelation Space trilogy, the Inhibitors are finally wiped out; however, it is implied that their absence is what allows a swarm of von Neumann machines to eventually consume literally the entire universe. As such this also counts as an Inferred Holocaust.
Ian Irvine, and the conclusion to the Well of Echoes series. The Magnificent Bastard has gained two artifacts of unsurpassed magic power and is taking over the world. Tiaan tries to stop him by destroying the power sources of all magic, thus preventing anybody from using it. Except then it turns out that said artifacts are the only significant exception, and what she actually did was destroy every single source of power that would have given the heroes a chance. Dang.
The Gun Seller, by Hugh Laurie. The whole plot is preventing arms manufacturers from pulling off a terrorist attack in order to jack up prices for their new attack helicopters... that made a lot more sense when I was reading it. But anyway, the last page states flat out that the sales of the very weapon he used to beat them skyrocketed. Awkward.
In Sailor on the Seas of Fate, Elric helps the Creature-Doomed-to-Live to die... And inadvertently sets in motion events leading to the end of the world. Oops.
In Kevin O'Donnell's novel ORA: CLE, set in a universe where all computers run unprotected operating systems like DOS and all news are shown in Bulletin Board Systems, the news is censored by a viral software implanted by the global Coalition. This is a perfect excuse for a coup by a group of erudites, who then keep the status quo and keep the censors; as the protagonist is an erudite but does not agree with the methods of the new group in power, he arranges to hack the several levels of censor programs, each more seriously defended, until immediately before deactivating the last one, he's warned by the leader of the group in power about not deactivating the last censor. The protagonist does anyway and the leader kills herself while communicating with the protagonist. Later is discovered that the censor programs were implanted to prevent extraterrestrial avian invaders, who use Earth as their hunting grounds, from finding out about a plan to transform Jupiter into a star in order to blind their sensors and allow Earth to launch everything against them in a last ditch effort to get rid of the invaders.
In the final arc of Deltora Quest, Leif and friends have to destroy the Four Sisters who are killing their land slowly by singing evil spells. No one hears their songs, as they are so quiet and have been in place for so many hundreds of thousands of years that everyone just thought of their songs as the sound of silence. When they finally manage to kill the last one, it turns out that the singing that was making the land barren actually also kept an even worse monster locked down: basically a bubbling pot of poisonous goo that will keep expanding until the entire land—the mountains, streams, forests, cities, and everything alive—is buried under a thick, hard crust of grey stuff. Essentially, it is unbeatable: swords cannot cut it, there is nowhere to throw it away and it expands too fast to curb it in any manner. The Shadowlord thought he had them beat there: die slowly or die quickly were the only options. Thankfully, the goo is not flame retardant, and they did have six gargantuan fire-breathing dragons on demand.
In the original novels of The Ring, the Asakawa delves into the mystery of the Cursed Tape not only because it's a good story, but to save himself, his family, and his friend from the killing curse. In doing so, he chronicles his investigation in the Asakawa Report, which details every little incident of the quest. By the end of Spiral, the second novel, Sadako reveals that the curse has mutated and taken on the Asakawa Report as its new vector, as well as any of its adaptations —movies, television, radio, and any other form of media where the tale is recounted. Eventually, all of mankind will have been destroyed as she replicates within each individual, infected human. At least the Cursed Video was contained...
Even in the original concept, it is only the protagonist who realizes that the only way to escape is by copying the Tape and showing it to someone else, as every previous instance stopped at the victim's death. However, by each film adaptation's sequel, the public at large is aware of this method: it doesn't take long for one to realize that this means the Curse will spread like wildfire throughout the world, especially with the advent of new media.
Four words, not in this order: 'cup','girls','one','two'...just wait until the effect suddenly shows up.
In the second book of Beyond The Spiderwick Chronicles, our young heroes succeed in luring the rampaging fire breathing giants into the ocean using a recorded mermaid song. As it turns out though, the giants were awakening so they could fight off an even bigger threat: a group of evil dragons that would be even more destructive.
In the third book of The Darksword Trilogy, Joram destroys the capstone of the Well of Life. While it seemed like a pretty good idea at the time, the fourth book describes a lot of the unpleasant consequences.
In Isaac Asimov's short story The Dead Past, the main character's research into time travel reveals that the government has been conspiring to hide the fact that new research means it is now easy to build a time telescope that can see perfectly anywhere in the world anytime in the last century and a half or so. The release of this information dooms humanity to existence with no privacy whatsoever, because you can just as easily set the time telescope to see 1/100th of a second ago as 100 years ago. Sometimes the government keeps secrets for good reasons, geniuses.
Were they protecting people's privacy or maintaining their monopoly on unbeatable undetectable Sinister Surveillance? Asimov should have written a follow-up detailing how ubiquitous time telescopes revealed all government corruption and solved all unsolved crimes.
Asimov didn't write that follow-up, but Arthur C. Clarke and Stephen Baxter did, in their novel Light of Other Days.
In another Asimov story, The Life And Times Of Multivac, humans chafe under the generally benevolent but definite control of Multivac. One of them figures out a way to maneuver Multivac into making itself vulnerable, then crashes it. The protagonist declares that humanity is now free... and realizes that it's not at all clear that freedom — including responsibility for running the world without Multivac — is what humanity really wants.
The Asimov story "The End of Eternity" has the Eternals (time travelers) doing this for all of humanity. In ensuring humanity had the maximum happiness for the most people, all tragedies were avoided. This also averted humanity's greatest triumphs. The long-term result was eventually, aliens took over the galaxy. Humanity drove itself to extinction over thousands of centuries as a result.
Metro 2033 has an extremely cruel version of this. To bottom line it, the protagonist ends up unwittingly eradicating a sentient and very humanoid species. Turns out it was trying to make peace with humanity all along and help it to survive. Oh, and it also kept a Sealed Evil in a Can in check, guarded the eponymous Metro from the worst hazards and was literally humanity's only and last chance to ever regain the planet... or even survive for a few more generations. Nice job indeed, Artjom.
Early in the first Hollows novel Ivy Tamwood is gifted with a wish. Rather than use it selfishly for herself she gives it to a Mia, a banshee who once gave her life altering advice. Banshees in the Hollows are the life draining apex predators of a world already filled with powerful monsters. Through various means Ivy's wish allows Mia to gain a human mate as murderous as she is, deceive people long enough to drain them of life before they can defend themselves and conceive a child more powerful than any banshee before her. No Good Deed Goes Unpunished indeed.
In The Company Novels by Kage Baker, Dr. Zeus the AI relies on the threat of this to preserve himself after his period of omniscience comes to an end. It doesn't work.
However, as the Rogue Cop Agrus Kos points out, that makes the first book into a no-win situation: The first book culminated in Szadek draining the leader of the Selesnya guild - the power sustaining the legal and magical obligation for all the guilds to exist. If Kos hadn't incapacitated Szadek, the power sustaining the civilization would have been destroyed, because he did, the resulting magical imbalance destroyed the civilization anyway.
In Neil Gaiman's Neverwherethis is basically the entire plot. All Richard has to do is help Door bring the key to The Angel, Islington, so It can return to Heaven and Door can be reunited with her family. Oh, Islington didn't mention that It isn't in Heaven right now because It had been banished to Earth by God? And its return would lead to God's will being subverted and war in Heaven, possibly leading to Heaven's destruction as well as humanity's? Oops.
A short story featured an American sniper recruited to test a prototype portable time machine. His mission? Go back some years before the 9/11 attacks and kill Osama bin Laden. Which he does, only to return to an America suffering a severe depression due to terrorist attacks using nuclear weapons, which he realizes happened because there was no bin Laden to desire to do something showy, but strategically minor. So he tries again, going back a little further. And again. And again. The end featured him in one of Pontius Pilate's dungeons awaiting execution due to a failed attempt on the Roman's life. (See also Hitler's Time-Travel Exemption Act.)
In Mitchell Scalon's Horus Heresy novel Descent of Angels, Lion (with Luther's help) unites Caliban to exterminate its horrific monsters, despite warnings that this might ruin Caliban. In Mike Lee's Fallen Angels, it is revealed that the monsters stemmed from Chaos taint, and so kept the people untainted, since they would avoid the monsters; killing them unleashed the taint.
In the backstory to Space Wolf: Ragnar's Claw the Wolves slaughtered an eldar army that turned up on a hive world. Unfortunately, the eldar were there because they were sealing away a Great Unclean One, and the Wolves killed them before they were finished so over the next several centuries the seal degraded until Botchulaz was able to send a plague out into the surrounding planet's population.
Codex Alera — The Vord? The pretty-much the ZergHorde of Alien Locusts that have almost eaten the entire world, and indeed already have conquered a continent much larger and more populated than Alera itself? It all began with one queen hibernating in an isolated valley until Tavi woke her up. He had no idea what he was doing or what the consequences would be, but that's cold comfort.
In The Ask and The Answer by Patrick Ness, Todd thinks he's saved an alien from death. Technically true. He's also allowed it to go raise an army so that said army can come back and kick everyone's ass. This was Mayor-excuse me, I mean President Prentiss's plan.
In Perdido Street Station, Isaac collects a staggering variety of winged animals for his study of flight, then gets fed up and releases or destroys all but one large caterpillar, which he feeds some of New Crobuzon's latest psychotropic street drug. It survives and pupates under Isaac's tender loving care ... and then emerges as a mind-devouring, hypnotic moth-monster that eats his roommate's psyche, escapes into the city, and frees others of its kind, which commence chowing down on every sapient mind they can catch. Nice work, Isaac. In fact, the monster ends up eating the mind of Isaac's girlfriend and leaving her a vegetable.
In the Dale Brown novel Air Battle Force, a Russian attack on a Turkmeni city is averted by destroying the bombers that would have carried it out; however, this spurs the Russian acting president, who was a bomber crewman and sees it as a personal slight, to carry out nuclear sneak attacks on the USA in the next book.
In Rogue Forces, a Turkish airstrike aimed at a Kurdish separatist recruitment drive kills the husband and children of former Kurdish separatist commando Zilar Azzawi, who retakes her sword in response.
The entire story arc of The Midnight Meat Train by Clive Barker. The protagonist ends up killing the butcher on the train, only to find out that his killing of the people were to feed the meat to demons as part of a bargain to keep the demon world from spilling over into the human world.
In Ted Dekker's Circle Trilogy, the entire plot of the first book involves the protagonist attempting to save two worlds and, consequently, bringing about the disasters (developing a biological weapon in one, letting evil back into the world in the other) he intended to stop. Nice job dooming two whole realities, hero.
Poliel is revealed to have begun the plague to purge the cancer of the Crippled God from Burn's flesh, until Paran and the Deragoth destroy her.
In The Kharkanas Trilogy K'rul sets out to create a new magic system that can be used by everyone, creating the opportunity for younger, mortal races to see eye to eye with Azathanai and Elder Gods. However, this new magic promptly makes the already fairly violent Tiste civil war into a powered conflict, with just a handful of mages on each side able to obliterate masses of people.
Pretty much the point of R.A. Salvatore's Forgotten Realms novel The Pirate King. The heroes lead a rebellion to free a city of the evil wizards that have been ruling it. They succeed, but leave large sections of the city destroyed, a good chunk of the population slaughtered, with a shortage of food and shelter and a long winter approaching.
Kitty Norville just wanted to help out a fellow lycanthrope who called in to her radio show. She had no way of knowing that she was actually enabling a psychotic serial killer who went on to kill four women before Kitty and Cormac put him out of his misery. Although, she can share that Nice Job with Meg, the Big Bad of the book, who infected James as part of a plan to assassinate Carl, then left him to fend for himself. Which eventually leads Kitty to expose Meg's scheme.
Defied in Rainbow Six. Homer Johnston is ordered not to kill a terrorist who would go on to kill a Littlest Cancer Patient because doing so would most likely lead to the other terrorists killing more children before the Rainbow teams can formulate and execute a plan to take them down. Understandably, no one is pleased about this.
In The Burning Realm, Kan Konar sets out to avenge the Deathlings by destroying Xoth, home fragment of the Cthons. He succeeds by destroying the Runestone of the fragment that shielded Xoth from sunlight, causing it to move aside, and forcing the nocturnal Cthons to flee their homeland or perish by the sun's rays. At the end of the book, Pandrogas discovers that this Runestone's destruction, coupled with previous disruptions caused by the Circle, has reduced the time remaining before the Shattered World's sustaining spells are exhausted from several decades to less than a year. Nice Job Breaking It Again, Hero.
In the original Dracula novel, the five male members of the True Companions insist that Mina Harker stay home while they do the dangerous work and frequently talk about what a relief it is that she's safe at home while they're hunting down the vampire because a woman surely couldn't handle it. Dracula deliberately takes advantage of this as an opportunity to bite Mina and metaphorically rape her. Nice job with the chivalrous misogyny, heroes!
In the Necroscope series almost every single victory the good guys have end up solving the immediate problem, but creating something far worse for the next book. Eventually they manage to "win" their way to ending the world.
At the end of the second book in the Star Shards Chronicles, Dillon brings back the Eldritch Abomination parasites from the first book, infects Okoya with them, and tosses him back through a dimensional vortex. Unfortunately, in the third book, it turns out the parasites run amok in the other dimension, displacing its powerful, soul-eating inhabitants, who are now forced to flee from their home dimension and decide to take over Earth. That turned out well, didn't it?
In The Pendragon Adventure, without naming the millions of times this happens, a notable mention should go to Bobby, who let the Big Bad nearly win because he quit, and it's slammed in his face at every opportunity just about until the end of the final battle.
In the first book, even though he's specifically told not to mix territories, he does it anyway, and the batteries in one of his gadgets are used to start a war, and even though it didn't last that long, everything ended up in ruin.
Mixing territories in general usually leads to this.
The plot of War and Democide Never Again is that two people go back in time in order to prevent all the worst crimes against humanity committed in the twentieth century from occurring. Technically, they succeed, but the sequel reveals that doing so allowed a dictator to rise the power when he had no such opportunity in the first universe, take over a major country, and nuke the entire rest of the world, setting mankind back centuries. Nice Job Breaking It, Heroes.
Robert Baratheon is determined to make sure that the exiled princess Daenerys does not survive to reclaim the throne with the hordes of Khal Drogo behind her. Drogo has no interest in thrones or the lands across of the sea... until somebody tries to assassinate his wife. Then he vows to sail across the sea, and conquer or destroy all of Westeros. Nice job waking the dragon, Robert.
Two more that have to do with Dany are when she picks a healer for her husband who was raped by her husband's men. This... didn't go so well. Drogo's dead. And speaking of that healer, she tries and succeeds in killing Khal Drogo, but in getting her revenge, she makes Dany more dangerous than either her husband or unborn son would have been by having her life-force used to hatch three fossilized dragon eggs. Oh, and teaching her that mercy is for wimps. Nice going, Mirri.
The Maesters of the Citadel have spent centuries declaring magic a spent force, denying the existence of many "mythical" creatures (or professing them extinct) and sneering at any who suggest that information pertaining to these things holds any merit. In fact, a fairly large faction seems dedicated to actively eradicating such silly beliefs. Which could shoot the Seven Kingdoms in the foot relatively soon, what with dragons and the Red God coming from the east, as well as Others decending from the north. With, perhaps, other "mythical" things joining in on the feeding frenzy, too. Oops.
This happens very frequently in The Dresden Files. Perhaps most notable in Ghost Story, where Harry realizes two different ways he's done this. Unleashing Mort's insane ghosts against Capiocorpus allowed her to devour them, manifest, then take any body she wanted and live again. In a wider sense, destroying the Red Court created an Evil Power Vacuum that brought out long-dormant monsters, some clearly more sadistic than the Court was, and engaging in open warfare to seize their resources. The first of these problems can be resolved fairly simply by destroying her, the second not so much. It gets worse. It turns out that Harry's assassination at the end of Changes was arranged by Harry himself, in an attempt to avoid having to serve Mab following his Deal with the Devil. He meant for it to prevent his (considerable) power from being used for whatever horrible ends The Fair Folk had in mind; instead, he wound up leaving his friends to deal with the aforementioned open warfare without their best source of firepower and his impressive reputation. Additionally, to keep Mab from realizing his plan, he had Molly erase his memory of it. Because of this, she's carrying around a ton of guilt for being complicit in his suicide, and the Leanansidhe has started her on Training from Hell; she's now such a wreck that none of her former allies trust her anymore. And it didn't even accomplish his goal. Mab and Demonreach managed to grab Harry's body and preserve it, so that when his soul should move on, he instead comes back to life... and is still in servitude to Mab. It's not hard to see why a fallen angel decided to push him onto that route, is it?
Also, there aren't many events in the series that aren't a direct result of Harry's refusal to abandon Susan to the consequences of her own actions in Grave Peril. This resulted in starting the war between the Red Court and the White Council eventually leading to the events of ''Changes'' and beyond.
This is what happens in Dark Sun when they kill the Dragon which had been plaguing the world. Turns out he and the Sorcerer Kings had been keeping a greater evil Rajaat imprisoned. Whoops.
Played with as Running Gag in Myth Inc. In Action, in which Guido and Nunzio enlist in the army to try to sabotage it from within. Every attempt they make to mess with its efficiency and operations, however, not only turns out to help the army, but gets them involuntarily promoted for their excellent leadership. Nice Job Not Breaking It, Hero!
Jack Vance's "The Miracle Workers" was set on a planet where human colonists made Hollywood Voodoowork to replace their aging technological weapons. When the planet's natives finally decided to attack the humans, one of the "jinxmen" noticed that dying natives spewed a purple foam . Deciding this foam must be associated in the aliens' minds with death, he used his powers to project the image of purple foam into the minds of a large group of natives. Another jinxman explained, "Then he learned that purple foam means not death—purple foam means fear for the safety of the community, purple foam means desperate rage." So he tried to intimidate them with an effect that turned them into Determinators. Oops.
Beetle's attack on the DoorKeeper in Queste, resulting in them being trapped in the House of Foryx.
Simon Heap should really have known better than to set the Things free.
In Warrior Cats, during the Omen of the Stars arc, Ivypool tries to protect ThunderClan from being invaded by ShadowClan by getting her Clan leader to declare war on them first. However, the entire invasion was made up by the Big Bad Tigerstar so he could weaken both Clans just before a harsh winter set in. Also during the same arc, Ivypool was spying on the Dark Forest in her dream, about to learn the plans for the final battle, when Dovewing woke her up.
Half Upon a Time ends with Jack, May, and Phillip accidentally releasing the Wicked Queen back into the world.
In The Magician's Nephew, the prequel to the Narnia series, the eponymous nephew is the one who accidentally brings the White Witch to Narnia after he succeeds in getting her out of our world, which he knew she planned to conquer - so he saved one world only to put another through a Hundred Years Of Winter.
In Mercedes Lackey's Owl knight, Darian and his company chase away the cold-drake that is blocking the pass to the home of Raven Clan, where his long-lost parents are living. Unfortunately, while this means that Darian can see his parents again, it means that the pass is now open to the marauding Wolverine & Blood Bear Clans. Everything works out, but it is a near thing.
Chanters of Tremaris see this happen twice, although the second is a subversion. In the climax of the first book, the heroes fight their way to Samis, only to find out that he used the spells they cast to reach him in his attempt to bring the all of the nine Powers under his control. In the final book, Marna breaks a relic in two so she can give half to Samis and convince him they don't have the rest so he will leave Antaris alone, and in doing so releases a plague (or so they think, for most of the book).
In the Wild Cards novel Aces High, Doctor Tachyon brings James Spector back from the dead as the result of an experiment intended to cure the Wild Card virus. Spector Came Back Wrong with the power to telepathically kill people by looking them in the eye. Spector, under the name Demise, becomes The Dragon to the Astronomer, a murderous cult leader bent on world domination.
In Galaxy of Fear this happens or is thought to happen more than once.
The Swarm: Zak accidentally kills a shreev, which he soon finds is one of the only things keeping the drog population in check. He hears a legend that the ecological balance is so delicate that one shreev killed out of turn will wreck it... and within days there are many more drog beetles than there should be. It's actually because a Beetle Maniac has been killing shreev by the thousands.
The Doomsday Ship: The ship's computer urges Zak to enter in some codes allowing it to take full control of the ship he's on, so it can help him. A hacker who'd been a Jerkass to Zak earlier pleads with him not to, saying the computer is evil. Zak, aware of the usual limitations of computers, punches the codes in. The hacker was right.
In the short story, Carnal Knowledge, a young man is pretending to be an animal rights activist so he can sleep with a combination Granola Girl / Soapbox Sadie he has the hots for. At the climax of the story, the two of them (along with her actual boyfriend he didn't know about) decided to break into a turkey farm and "liberate" all the turkeys before they're slaughtered for Thanksgiving. However, since the turkeys were bred in captivity and didn't really understand the concept of freedom (and aren't the brightest birds anyway,) they didn't really "fly to freedom" as much as they waddled onto a nearby highway and got pulverized by an oncoming eighteen-wheeler.
In the T2 Trilogy by S. M. Stirling, John Connor breaks into a secret military base in Antarctica. He finds the computer on which Skynet is currently running. His tech geek girlfriend's just installed the code that will suppress all A.I. Success is in his hands. Unfortunately, he doesn't realize that his girlfriend didn't install the part that actually blocked the A.I. from working. The instant he presses Enter, he's set Skynet into motion.
Between them Húrin and his son Túrin end up breaking pretty much all of Beleriand that stands free after the Battle of Unnumbered Tears. Húrin is captured by Morgoth and his son cursed, and due to the curse (or his own pride), Túrin ends up leading the Elves of Nargothrond to destruction and killing the last chief of the Haladin. Húrin is released after his son's death, and ends up inadvertently leading Morgoth to the location of Gondolin, and brings a cursed piece of jewelry to Doriath, which leads to the death of Doriath's king Thingol (the beginning of a chain of events leading to its destruction).
The Sons of Fëanor (most notably Celegorm and Curufin) also constantly sabotage their quest to defeat Morgoth and retrieve the Silmarils. By slaughtering the Elves at Alqualondë, they turn King Thingol against them, and further alienate him when they kidnap his daughter. They also try to usurp the throne of Nargothrond, which leads to the Elves there shunning them. These factors mean neither kingdoms send any significant number of troops to the Battle of Unnumbered Tears, helping their defeat.
Refusing to kill Jihidain in the past is the reason she is still around to cause trouble.
Refusing to train Benji means he jumps into danger without the skill necessary to get out of it or the self-disciple to stay out of it in the first place. This almost gets her and Lydia killed.
Refusing to tell Benji that his Aunt Zarracka is evil means said aunt has no trouble convincing him that she is good and Daniar is the bad one. This gets a city trashed and Final Shield destroyed.
In the last book of the Earth's Children series, "The Land of Painted Caves", Ayla reveals that men help make babies. The men immediately turn into patriarchal jerks who want to own their children. Nice job breaking it, Ayla.
He said, "Think. Who allowed the outbreak to occur? Who failed to report the appearance of the parasite to any authorities? Who prevented any containment at your house? Who created the breach at the REPER command center? Who created the breach in the quarantine containment fence? Who single-handedly spread this infection?"
John said, "We didn't do any of it on purpose. We're just... not very good at things."
In Croak, the Big Bad only gets the ultimate power because Lex tries to use it on her, allowing her to "Cull" it.
In Caryl Ferey's Zulu, the protagonist, Ali Neuman, is on a Roaring Rampage of Revenge. He tracks down the Big Bad and The Dragon in the middle of a desert. Ali manages to incapacitate the Big Bad and kill The Dragon by ramming his car into theirs, which sets off a massive explosion. So far, so good. Unfortunately, this leaves Ali and company alone in the desert, miles away from any habitation, with no water and no means of escape...
Captain Underpants: The plots tend to start with George and Harold playing a prank on their cruel teachers, only for it to spin out of control and create the main conflict of the book. They eventually realized this and resolved to be good kids...which ended up creating the conflict anyway.
In Those That Wake, Mal leaving the door open at Mike's school let hopelessness infect it. In the sequel, Rose gives the Old Man access to the neuropleth, an action which leads to Mal's death.
The U.S. Government in Day By Day Armageddon decides the best way to contain the zombies is to nuke every major city. While this does kill thousands upon thousands of zombies (and survivors) any zombies not vaporized by the blast or humans killed by the radiation or fall-out become "Joggers": zombies that don't rot, are much faster, display problem solving and the ability to use weaponry, and can kill you just by getting close because of how radioactive they are.
Arguably, God in The Christ Clone Trilogy. The Trumpet Judgements devastate Earth and demoralize humanity to the point where the Antichrist is poised to seize power. Without them, it's not likely Christopher would have risen to power as he did. Then later, the Bowl Judgements give Christopher the pretense and popular support to step up persecution of Christians and Jews.
The Newsflesh world gives us Brandon Majors, stoner, legend in his own mind, and leader of a "radical" group styling themselves the "Mayday Army". After reading a deliberately inflammatory article by muckraking "journalist" Robert Stalnaker (who also qualifies for this trope), Majors and his cohorts storm a medical research lab to release an engineered virus intended to cure the common cold. When this meets another engineered virus (this one a cancer cure), hello Zombie Apocalypse.
Lucy from The Sex Lives Of Siamese Twins saves two homeless men from being killed by a deranged gunman. Turns out, she just managed to save two serial pedophiles from a broken victim of child sexual abuse.
The Pies are having trouble selling rocks... because after the Crystal Empire was saved by the Mane Six, it started a jewel craze.
Pinkie's parents ask Pinkie to be more serious, and it causes some major issues for a good portion of the book.
In the first Pandora Jones book, Pandora's discoveries lead her to conclude that the virus is fake, the world hasn't ended and the whole School thing is an elaborate lie. In the second book, her attempts to find out just what the fuck is going on leads to the revelation that the world hasn't ended, true, but the virus is real and everyone at the School has it. Originally, the people behind the whole thing were going to infect the world later, but Pandora and Jen's efforts led to them accidentally infecting some outsiders, so in essence, Pandora kickstarted the apocalypse by accident and in the coming days, she'll have accidentally caused thousands, if not millions of deaths.
In Heart of Steel, Alistair's attempts to remove Jim as a romantic rival for Julia's affections (not knowing that she had been planning on breaking up with Jim) instead creates a crazy, jealous cyborg monster hell-bent on killing both of them rather than lose Julia.
In Coiling Dragon, Linley finds a sword sitting in the middle of a magical construct. Since it must be powerful, Doehring tells him to take it. After he leaves, a deity emerges from the prison plane through the weak point that the sword had kept shut. That god then leads a magical beast army to destroy the kingdom's capital. Later on, more gods escape from that weak point and murder millions of people. When Linley, Desri, and Oliver are fighting one of them, a second, bigger hole into the prison plane is opened. Many, many more Saints and deities escape.
In the Revanche Cycle, Amadeo comes up with a grand plan to rescue the imprisoned Livia, and rallies the downtrodden souls of the Alms District to her aid. Problem: Livia had a plan of her own, and didn't need his help. And the reprisal in the wake of Amadeo's "rescue" leads to the Alms District in flames, and several hundred innocent people murdered.
In the Doctor Who novel "Engines of War", the Time-Lords tried altering the course of Dalek history. Giving the Daleks the idea to try and alter the course of Dalek history, creating the various Skaro Degradations.
The Hunger Games: Katniss' main goal through the second book is to find a way to trick Snow into believing she's in love with Peeta. Unfortunately, she does convince him (and everyone else), and therefore manages to give him the leverage to break her during Mockingjay. And you can say that the entire series is this. Prim dies anyway, which was what the instigation of the plot of the first book was trying to prevent.
In A Time To Die, Claudia Montierro is on safari with her father Ricardo and protagonist Sean Courtney, when she tries to interrupt an attempt by her father to shoot a lion, not wanting the creature to die for sport. The lion is subsequently wounded, forcing the rest of the party to risk their lives to put it out of its misery. Chaos ultimately ensues when the lion and his mate attack the hunting party, during which one of Sean's trackers/True Companions ends up crippled and the lioness gets killed, thereby dooming her unweaned cubs to a slow death by starvation and putting Sean in danger of losing his concession. As a furious Sean points out to Claudia later, if she hadn't interfered, the lion would have died quickly and painlessly without anyone else having to suffer.
In Tales of the Magic Land one of the Woodcutter's first deeds as sovereign is pulling down a sinister-looking high, thick thorned iron castle wall, left like this by his Big Bad predecessor. Three books later, an evil giantess attacks the country and almost makes it to the palace there's nothing to block her way.
In Starlight and Shadows, Liriel wants to use a powerful magic artifact called the Windwalker to carve a rune on Yggdrasil's Child that will allow her drow magic to function on the surface and endure the sunlight. The evil drow goddess Lolth seizes upon the magic of the rune and strengthens it so that all drow are able to use magic on the surface, furthering their plans for conquest.
In the Relativity story "Candy Corn", a pair of incompetent criminals manage (through sheer luck) to capture one of the superheroes. Upon regaining consciousness, the hero mocks the criminals for not taking off his mask while they had the chance. They immediately remove it.
In The Happy Return he follows his orders to reach the west coast of Central America without sighting land (a nigh-impossible navigational feat) to support rebels against Spain, and thus has no idea that during those months England signed an alliance with Spain. He doesn't learn this until after defeating a Spanish warship and handing it over to the rebels. He then has to fight it a second time and winds up sinking it, which pisses Spain off even more.
In The Commodore, he's given an interpreter to deal with the Russians and Scandanavians. It is not his fault that this interpreter has a hugely murderous grudge against the Czar and the King of Sweden and thus inclined to steal Hornblower's custom-made, very accurate, and easily identifiable pistols to kill them... but it's certainly where the blame would have fallen if he hadn't thwarted it in time.