Metal Beard: If you wish to do harm to yon wedding, you'll have to get by we, first!
The Conflict of the story expands (even if for a short time) from merely stopping whatever antagonist the heroes were against, to stopping one of the Hero Protagonists, who is making a serious mistake.
In this trope, a greater or more sinister problem is compounded when one of the characters realizes that something is amiss: namely, that either what the hero has done is wrong all along, or he's about to unwittingly do something wrong. This trope is a Plot Twist that comes in an arc/conflict and reveals that someone on the "right" side is doing something wrong. It isn't that they reveal themselves to be Evil All Along, there's just some major piece they're missing or ignoring, or they've become momentarily blinded by their impending victory, or maybe they've been an Unwitting Pawn in a greater scheme. If someone doesn't stop the Hero, or warn them, or save them, then things are about to go From Bad to Worse.
This trope is more common with Anti Heroes, as they're generally uninterested in a morally-perfect resolution, and are simply getting the job done or their own goals fulfilled by any means necessary. However, sometimes it's The Paragon or The Cape who are the ones about to screw up, as it demonstrates that even great heroes can make mistakes. In many plots, this last revelation/challenge is the final test the hero needs to complete their Character Development, often with a Get A Hold Of Yourself Man or My God, What Have I Done? moment.
This trope isn't "that time the hero messed up". For it to apply, the actions of the character must 1) take place within the main story and 2) potentially have disastrous results which affect the story's outcome. Naturally, how this trope is played and how close we are to the finale determines what type of ending you get. If the Hero is stopped, or realizes their mistake soon enough, then it's possible to still get a Happy Ending. If they're not, or no one notices the fault soon enough, then the story might be doomed to a Bittersweet Ending at best and a Downer Ending at worst. This is also a very common ending to a Protagonist Journey to Villain.
Polar opposite trope of Save the Villain, although the two can intertwine. If the villain now needs to do the saving, it's Villainous Rescue. If the hero isn't stopped, it shifts to Nice Job Breaking It, Hero!.
May involve Treacherous Quest Giver; Cycle of Revenge; Not Quite the Right Thing; If You Kill Him, You Will Be Just Like Him; My Death Is Only The Beginning; No Good Deed Goes Unpunished, and You Kill It, You Bought It. This trope is usually the secret motive of the Poisonous Friend.
Disclaimer: As this marks a major turning point in the story and can also happen near the end, there might be unmarked spoilers.
- Bleach: In the Fullbringer Arc, Ishida is attacked by an unknown assailant and winds up in the hospital. Meanwhile, the rest of Ichigo's companions also find themselves under attack, and soon a new adversary named Tsukishima is revealed. Ichigo, Brought Down to Normal after his previous battle over a year ago, is desperate to help his friends in some way and Sado introduces him to a group of humans with spiritual powers called "Fullbringers" led by a man named Ginjo. The Fullbringers train Ichigo and help him gain new powers, but in the meantime, Tsukishima attacks the rest of Ichigo's friends and family—revealing that his sword's powers are a form of Mental Time Travel that alters someone's past—and he uses this to make the people he attacked think he was their friend and The Hero instead of Ichigo, turning them against him. Ginjo and Ichigo, the only two people left unaffected, battle Tsukishima until Ginjo finds himself cut by Tsukishima's sword and urges Ichigo to end the battle before the mental effect takes hold. Ichigo attacks Tsukishima once more, only to be interrupted by the arrival of Ishida, who pulls his weapon on Ichigo. Ichigo assumes that Ishida was brainwashed like everyone else, until Ishida yells that Ichigo has it all wrong—the person who attacked him wasn't Tsukishima...it was Ginjo. It's then revealed that Ginjo is the mastermind, that being cut just undid the brainwashing he was already under, and that he had willingly let Tsukishima turn him into a Manchurian Agent to dupe Ichigo.
- Dragon Ball: In an anime-only battle, Goku tries to stop a resurrected Vegeta from fighting the immensely-powerful Super Buu on his own, knowing that fusing with Vegeta is the only hope and that Vegeta is the last person still alive whom he can fuse with. Vegeta, however, ignores Goku and continues to almost get himself killed against Buu, finally yelling that he'd rather die than fuse with Goku because Goku held back against him in their earlier duel. Goku has to call Vegeta out for putting his pride and birthright above protecting their friends and family for Vegeta to finally give in and allow the two of them to fuse.
- Fairy Tail: Zig-Zagged during the Grand Magic Games arc. Natsu's group find out that Princess Hisui Fiore plans to open the Eclipse door to stop a horrible calamity, and they tell her that they have her back. However, Future Rogue appears in front of them to kill Lucy on account of her closing the door. Lucy says that she wouldn't do such a thing. After helping Hisui in opening the door though, Lucy gets info about what's behind the door from her celestial spirit, Crux, and she tells everybody that they need to close the door due to it being a gateway to the past where dragons roamed rather than a cannon like Hisui thought it was.
- Rebuild of Evangelion: The climax of 3.0 revolves around the pilots of WILLE trying to stop Shinji Ikari from accidentally triggering Fourth Impact, which will finish the job that the partial Third Impact he accidentally triggered in the second film started, in his drive to fix things thanks to his immense guilt complex. Nice Job Breaking It, Hero! is occuring on both sides, even — while Shinji is to blame for the Impact, everybody who works for WILLE couldn't stop being assholes to him for just long enough to provide him with a detailed explanation of just what the hell is going on (the best they provide him is "You Know What You Did" even as he makes clear that he doesn't), and in his drive to obtain answers he went back to NERV and his Manipulative Bastard of a father who then turned him into an Unwitting Pawn.
- JLA: Earth-2: The Justice League tries to fix the problems on the Antimatter universe, only to find out that said world's natural order is for evil to triumph over good, so in fact, they're doing a disservice to the people they want to save simply by being heroes. This is demonstrated at its strongest when Batman saves Police Commissioner Thomas Wayne - father of his Owlman, his Evil Counterpart note - from a man who was attempting to assassinate him, only for Commissioner Wayne to announce his intention of making Gotham into a Police State. The League then desperately tells one another of the problem, and that the two worlds will collide.
- Civil War: The miniseries ends with a group of ordinary people restraining Captain America during his fight with Iron Man, because their fight was smack in the middle of New York City, and causing untold damage. At the time, Iron Man was the figurehead of a movement to make superheroes accountable for actions like this, so those civilians attacked Cap because they saw him as being part of the problem. Cap himself is remorseful for his actions and surrenders.
- The LEGO Movie 2: The Second Part: The movie at first follows Emmet as he tries to save his friends from aliens with the help of a space explorer named Rex Dangervest. Meanwhile, Lucy tries to fight the aliens who abducted her, but in doing so, she discovers that the aliens are not hostile and want to make peace between the two worlds. Then she realizes that Emmet is about to ruin everything with a Megaton Punch, manipulated by Rex, who is the real bad guy.
- Avengers: Age of Ultron: Midway through the film, Cap and the Maximoffs, after having just defeated Ultron, team up to stop Tony and Bruce from experimenting on Ultron's dormant Vibranium body, as it resembles the initial experiment that created Ultron in the first place. Subverted, though, when Thor shows up: everyone expects him to smash the body, but instead he brings it to life with his lightning, creating the heroic Vision.
- The Dark Knight: When Batman heads to confront the Joker, he discovers the villain has disguised his goons as prisoners and the prisoners as goons, meaning the SWAT units going in will likely shoot the wrong people. He thus takes a detour to punching out the SWAT cops before they can do any harm.
- Terminator 2: Judgment Day: At the start of the film's third act, Sarah Connor goes on a mission to assassinate Miles Dyson, the Unwitting Instigator of Doom that will cause the robot uprising and near-extinction of the human race. When her son John realizes that his is her goal, he immediately rushes off with the good Terminator to stop her. They don't actually make it in time, but Sarah realizes that she's become exactly like the Terminators she despises and sparing Dyson allows the heroes to invade Cyberdyne to destroy Skynet once and for all.
- X-Men: First Class: At the film's end, Erik Lensherr kills his nemesis Sebastian Shaw despite Xavier's attempts to stop him (forcing Xavier to feel all the pain of the mentally-controlled Shaw as he's killed) and, after the US and Soviet armies attempt to turn on the mutants with a missile bombing, stops said missiles and fires them back at the gunships. It's at this point that Erik's Start of Darkness is complete, and Xavier knows that if he doesn't stop Erik from killing these soldiers, it will cause an apocalyptic war between humans and mutants. Erik winds up stopping when he deflects one of Moira's bullets and it strikes Xavier in the spine.
- Isaac Asimov's Foundation Series: In "The Mule", the heroes spend Part Two of the story trying to find and contact a secret organization which is their only hope against the Mule. Ebling Mis manages to figure out the location, is about to reveal it... and is shot by Bayta Darell, because she realized the Mule is one of their group, has been all along, and is about to hear the answer.
- The Dresden Files: The short story Backup, which is told from the perspective of Thomas Raith rather than series protagonist Harry Dresden, involves Harry being used as an Unwitting Pawn and Thomas having to stop him without letting him know what's going on, as the beings responsible become more powerful the more people know about them.
- Mistborn: The Original Trilogy: At the end of the second book, Vin realizes too late that she's been an Unwitting Pawn of the Piece of God Ruin, who manipulates her into releasing it from its prison despite Preservation's last-ditch attempts to stop her. The third book consists of Vin and her companions' desperate efforts to engineer a Pyrrhic Victory over Ruin.
- Farscape: In "They've Got a Secret", Moya has seeming turned against the crew and they make the difficult decision to lobotomize her for their own survival. Then they discover that Moya is pregnant and just protecting her unborn child, and must call off the lobotomy over a poor-quality radio link.
- Supernatural: Throughout season 4, the Winchesters, under contract from Heaven itself, have been fighting demons led by Lilith to stop them from breaking the seals to Lucifer's cage. By the season finale, "Lazarus Rising", Dean discovers that the angels have been lying to them and want the Apocalypse to happen. Sam, who is on his way to kill Lilith, will inadvertently be the cause of the Apocalypse because Lilith IS the last seal. Despite last-minute help from the rebel angel Castiel, Dean is too late to stop Sam.
- Art of Fighting: In the end, Ryo has defeated the last boss Mr. Karate and is about to beat him up more for info on his missing sister. Suddenly, said sister Yuri jumps out and shouts for Ryo to stop, saying that Mr. Karate is actually their missing father, Takuma.
- Dead Space 3: In the Awakened DLC, Clark and Carver have destroyed the Brethren Moon that was almost completed in the game proper and are looking for a way to return back to Earth when Clark starts to worry that escaping to Earth will let the rest of the Brethren Moons know where Earth is and provide them with the means to wipe out the human race. Carver believes Clark is having another hallucination caused by the Moons and the two fight. The fight is interrupted when the Moons reveal that they knew where Earth was already, and that Carver was right: by wasting time, Clark has given the Moons plenty of time to beat them to Earth and start their invasion. The ending (assuming it isn't another hallucination) shows that said invasion has begun as the Moons envelop the Earth.
- Devil May Cry 5: After revealing that the newly-resurrected Vergil is Nero's father, Dante leaves to fight Vergil in a final duel that will determine which of the two twin brothers will emerge victorious in their long rivalry. While the other heroes argue to just leave things to Dante, as it would be cruel to make Nero kill his own father to stop The End of the World as We Know It, Nero feels like none of this makes any sense and won't allow anyone to die, including Vergil, over something so petty. Thus, he finally awakens his matured Devil Trigger form and drops between Dante and Vergil just before their final blows connect, knocking Dante for a loop with one blow and then declaring that everything will be settled for good if he defeats his father. He does, and the two men stop fighting long enough to enter the Underworld and close the gate—Passing the Torch to Nero to protect the world in their absence.
- Final Fantasy VII: In the Northern Crater, main character Cloud has an existential crisis when he finds out that memories he had of revisiting his hometown years earlier are fake. The villain, Sephiroth, takes advantage of this to convince Cloud that he is a copy of someone long dead. Cloud, confused and feeling hopeless, does not resist Sephiroth's prodding and decides to fulfill his "purpose" by retrieving the Black Materia from his teammates and handing it to Sephiroth. Tifa tries desperately to call out to Cloud and to stop her team from giving him the Black Materia, but it's to no avail. Sephiroth immediately uses it to start The End of the World as We Know It and this causes the bleak second half of the game to transpire.
- Final Fantasy X: The female lead, Yuna, is on a quest to defeat Sin, a giant Kaiju that has the power to destroy entire cities in an instant and would destroy the world if left unchecked. Throughout the game, a despised group of outcasts called the Al Bhed continuously try to kidnap Yuna and impede her quest to obtain the "Final Aeon". Later, it's revealed that this is because the quest itself is flawed. Even if destroyed, Sin will come back in ten years or so and, far worse, this will also kill Yuna herself. Yuna's party knew and accepted this, but protagonist Tidus is shocked and vows along with Yuna's cousin Rikku (one of the aforementioned Al Bhed) to find a way to save Yuna. There is a faint hope that one day, Sin's death will actually stick, and Yuna is adamant that it must be destroyed no matter the cost regardless, so they press on right up to the final portion of the journey, where the party learns one last Awful Truth: contrary to what the masses believe, this method will never defeat Sin permanently, and the ritual actually sacrifices two people: the summoner and someone close to them. When Yuna learns this, she rejects the pilgrimage outright and destroys the entity that creates the Final Aeon, thus leaving the world no other option than to find another way.
- Golden Sun: The first game has you pursuing the bad guys and their hostages to prevent them from lighting the Elemental Lighthouses, as doing so would cause the return of Alchemy to the world and therefore its destruction. The second game instead has you control said hostages and actively light the beacons until you're reunited with the heroes of the first game. Unbeknownst to the party of the first game, the return of Alchemy wouldn't necessarily spell the end of the world (it would certainly have the potential to do so in the wrong hands, sort of like going from no nation having nukes to every nation having them), but leaving it sealed will cause the world to crumble away. The villains of the first game are retroactively revealed as Well Intentioned Extremists whose village was threatened by the world eroding, and took a particularly ruthless course of action to save it.
- Halo: Combat Evolved: Halfway through the game, the Marines think they've found a weapon to use against the Covenant, but Cortana accesses the files of the Halo facility and learns something which terrifies her. She immediately tells Chief that he has to stop the Marines before it's too late, but he fails and this results in the release of The Flood, the horrifying menace that the Halo was created to contain and destroy in the first place and is a greater threat than the Covenant aliens ever were.
- Life Is Strange: The climax of Season 1 basically consists of Max and Chloe coming to the realisation that Max using her powers to save Chloe at the beginning is the cause of the storm that's about to destroy the town; Chloe tries to convince Max to go back and just let her die. The player gets to choose if Max does so, or Screw Destiny and sacrifice the town for Chloe.
- Mortal Kombat 9: The Downer Beginning starts with Raiden using Mental Time Travel and telling his younger self "He must win". The rest of the game centers around Raiden trying to figure out whom "He" refers to, with such dire consequences that Shao Khan winds up killing almost all of the heroes and achieving victory. Liu Kang becomes fed up with Raiden's mistakes and resolves to kill Shao Khan himself...only for Raiden to realize that "He" refers to Shao Khan. Raiden tries to stop Liu Kang from killing Shao Khan, but this causes Kang to become even more convinced that Raiden has lost his mind, resulting in a fight that ends with Liu Kang's death as well. Afterwards, Shao Khan tries to merge Earthrealm and Outworld, whereupon he is promptly killed by the Elder Gods, because they have very strict rules against merging realms.
- Shadow of the Colossus: Wander follows the instructions of an unseen voice telling him to slay the sixteen Colossi roaming the lands. He's told that doing so would bring his loved one back to life. As the game progresses, we see there is a group of priests who are traveling to the land, and Wander eventually discovers that each Colossus was a living seal keeping the Ambiguously Evil Dormin sealed away. Defeating the final Colossus results in Dormin breaking loose and possessing Wander's body just after the priests arrive to warn him of what he was doing. From there, Dormin eventually gets sealed back in, and the girl awakens to find a little baby with horns nearby.
- Tales of Symphonia: Throughout the heroes' adventure to regenerate their world from centuries of decay, they are continuously attacked by a mysterious assassin trying to kill Collette, The Chosen One. Near what's supposed to be the end of their journey, they learn that the assassin, Sheena, is from another world that's connected to this one, and if the heroes are successful, her world will start to decay like their own. This revelation sparks a far more expansive adventure where they learn that the journey of the chosen was a big lie perpetrated by the bad guys behind the original separation of the two worlds.
- The Batman's "Strange New World" sees zombified versions of the GCPD and Batgirl, later joined by Robin, try to stop Batman from curing Gotham after Hugo Strange claims to have just released a toxin into Gotham City and dose Batman and Robin with a vial of what he was said was an antidote. It isn't until the climax, when Bruce pieces together various things, including the Batcomputer having detected no toxin in the air and the timing, that he realizes the "cure" he and Dick were exposed to was really a hallucinogenic drug and Strange played him into almost releasing the real toxin, allowing the GCPD to administer the real cure.