"Watson, if it should ever strike you that I am getting a little over-confident in my powers, or giving less pains to a case than it deserves, kindly whisper 'Norbury' in my ear, and I shall be infinitely obliged to you."
Nothing defines a hero better than his morals, and the biggest sympathy point can be guilt over some monumental screw-up that taught the hero to buckle down and stop taking his job too lightly. This is sometimes related to the origin of a Super Hero, but has more impact when the hero's career has otherwise been going well for a while.
From a philosophical standpoint, this makes sense—if someone wins almost all the time, as most comic book heroes do, they would be more defined by their failures than their successes. In Marvel Comics, this is sometimes a consistent psychological flaw (the "Marvel Flaw") which occasionally prevents a hero from succeeding.
The hero will often declare "It's All My Fault" while their friends and family say "You Did Everything You Could."
The hero may recover from this and when they do, it's usually a sign they have grown, although retcons can cause an unpleasant return to status quo.
The Failure Knight has this as part of his backstory to explain why he is so devoted to his new charge. Often produces a combination of Bad Dreams and Anxiety Dreams.
See I Let Gwen Stacy Die for two of the more common failures. Might result in We Used To Be Friends. Often happens to heroes who fail to make amends. Can be a Career Building Blunder. When the failure is what put the character on the path to being the current (better) person s/he is now, it's Necessary Fail. If the plot brings about an opportunity to correct or make up for their failure, you have My Greatest Second Chance.
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Anime and Manga
To Naruto, his not being able to save either Sasuke or Gaara is this, especially in the infamous Tear Jerker where he thinks he is useless for not being able to save either. Thankfully, Gaara gets better, at least in the physical sense.
Jiraiya looks back on his life as a long series of failures, but the one that most stands out is failing to stop Orochimaru's descent into evil.
Kakashi views most of his life as this: he failed to save Obito and Rin, he couldn't stop Sasuke from betraying Konoha, and he couldn't be Obito's eyes.
With the revelation of Chapter 599, his failure to save Obito and Rin is probably what he considers his absolute worst failure by far.
Sara's post-traumatic stress syndrome for most of Soukou No Strain is only worsened when Carris dies too.
For Hon Arkraptor from Tower Of God, his inability to save his daughter is what drives him to climb the Tower.
In Fullmetal Alchemist, Edward Elric feels terribly guilty for pushing Alphonse to help him use forbidden alchemy to try and resurrect their mother Trisha, leading to the loss of Ed's limbs and Al's human body. It takes years before Edward gets up the courage to ask if Alphonse blames him for it, but the answer is ultimately no; Alphonse even says that he knew the risks and went along with it.
The Elric brothers also consider their being unable to save Nina Tucker from being turned into a chimera, or dying shortly afterward, a massive failure, as well as proof that they are ultimately mere humans.
Yu-Gi-Oh! GX: Season 3 has been one big Greatest Failure for Judai, ironically originating from his insecurities about never failing.
In Rurouni Kenshin, Kenshin is still wracked with guilt over, among other things, not being able to protect his first love, Tomoe. This is magnified in the manga's Jinchuu arc when he apparently fails to protect the woman he loves a second time (after Enishi fakes Kaoru's murder). He eventually recovers, though, and finds out she's still alive as a reward.
A lot of the backstories of characters from One Piece involve this: Chopper not being able to save Dr. Hiriluk, Franky failing to save Tom from the World Government, etc.
Kalos Eido from Kaleido Star blamed himself for more than a decade for asking his old friend Aaron Killian if he really was up to perform the Fantastic Maneuver, thus (inadvertently) planting a seed of doubt in Aaron's mind that caused him to fail and die; this still haunts Kalos up to this day, thus making him vulnerable to Smug Snake Yuri's manipulations... since Yuri is Aaron's son.
It seems that not being able to fully save Ace will haunt both Luffy and Jinbei for years to come.
And Luffy had one not long before that — the defeat that scattered the Straw Hat Pirates at the Sabaody Archipelago.
Heero Yuy from Gundam Wing, as revealed in The Movie, accidentally destroyed a building while bombing a military base, killing (at least) a little girl whom he had befriended the day before. This put a face on his victims and made him The Atoner until his encounters with Rebellious PrincessRelena gave him something to fight for.
Gundam 00 has quite a lot of these: there's Sumeragi's guilt over causing a friendly fire incident that killed her lover (accidentally: she was given false intelligence), Smirnov's guilt over (indirectly) killing his wife through following his orders, and Setsuna seems to consider murdering his own parents in cold blood as a child to be his. Then there's that whole incident with Saji and the Katharon base...
Mobile Suit Gundam Seed has this a couple times with Kira Yamato. The first time being the failure of saving a shuttle full of fleeing civillians, one of them being a little girl he recently befriended. The second time was when he failed to save Flay Allster, his first love.
And to a lesser extent, Yzak Joule's failure of losing to Kira Yamato. His face was badly scarred during a battle, that was inevitably lost. Though he could of removed the scar with surgery, he chose to keep it until he settled his score with Kira.
Chichiri from Fushigi Yuugi has spent years trying to atone for the circumstances leading up to his best friend's death. This inspired him to become the calm, dedicated monk we're first introduced to when the series begins.
In Mahou Sensei Negima!, Setsuna's Failure Knight nature has its roots in an incident where she tried to save Konoka from drowning, fell in herself, and both were rescued by someone else. As a result, she constantly tries to train harder so it won't happen again. Konoka doesn't hold it against her, as they were both young children when this occurred.
In Magical Girl Lyrical Nanoha A's, Admiral Graham blames himself for Admiral Clyde's death, as he was forced to destroy the Hestia with Clyde on board, at Clyde's insistence, rather than let the Book of Darkness take over the ship and use the Arc-en-ciel on the other ships.
Minor character Vice also has one of these: he used to be a sniper of quite a bit of skill, but he now pilots helicopters. Why? Because several years back, his little sister was involved in a hostage situation. He tried to shoot her kidnapper but missed... and hit her in the eye. Though she survived (and doesn't hold him responsible, recognizing it as an accident), he cannot forgive himself, and resolved to give up sniping.
At some time between Striker S Sound Stage X and Force, Subaru was involved in fighting a fire, but despite her efforts, there were many deaths. She then took a leave of absence to train, resolving to become strong and fast enough to save everyone.
In Bleach, Urahara's failing to save the Vizards was this.
In Durarara!!, everything Shizuo Heiwajima hates about himself can be distilled into one incident during his childhood: when he was about ten or eleven, he developed a Precocious Crush on a woman who looked out for and worried about him due to his constant injuries (unaware that they were the result of overexerting himself in violent, uncontrollable bouts of anger). One day, while walking by her store on the way home, he saw her being assaulted by Yakuza thugs. Shizuo tried to help. He screwed up. While this wouldn't be the last time he'd end up hospitalizing someone he was trying to protect (the light novels imply that this sort of thing happened constantly), it's the example that sticks with him the most.
In Yu Yu Hakusho, Younger Toguro is a Death Seeker because he felt he couldn't suffer enough after a demon killed his students.
The Legend of Koizumi has The Pope turn out to have unknowingly snuck Hitler out of Germany back in '45. He has, for reasons that should be blindingly obvious, regretted it ever since he learned what he had done.
In Sword Of The Stranger, Nanashi has more or less sworn off drawing his sword. What makes this interesting is that his failure is MORAL rather then Physical. He executed two children, by power of peer pressure. It's hard to disobey orders when surrounded by an army. Part of a military action, it's unclear the exact circumstances but it was likely a coup d'état. Nanashi only draws his sword in the eleventh hour in a last desperate attempt to save Kotaro, who strongly resembles the boy Nanashi executed. By BITING THROUGH THE PEACE KNOT WHILE RUNNING AT FULL SPRINT ACROSS ROOFTOPS IN THE SNOW TO THE CROWNING MUSIC OF AWESOME.
In Holyland, Masaki's is begging forgiveness from some thugs when he could have stood up to them and took them down. Yuu treats Shinichi getting attacked this way.
In Busou Renkin, after seven years, Captain Bravo still cannot come to terms with the Alchemist Warrior's greatest - and only - failure where a school was destroyed and devoured by homunculi, save for one girl: Action Girl Tokiko.
Dragon Ball's Kami was the one who created Demon King Piccolo, the latter being formed from the former's inner evil. As such, Kami believes that he's as responsible for Piccolo's misdeeds as much as Piccolo is;
Kami: I should have given up being a god quite some time ago... after all, it was I who brought the fearsome evil known as Piccolo into this world. What right have I to be a god after this? What right have I to go on living?
In Berserk, not being able to save Casca from being raped by Femto during the Eclipse and later realizing that he made a big mistake by leaving her for two years to deal with her trauma by herself while he pursued revenge against Griffith in order to deal with his trauma alone was Guts' greatest failure.
In the horror manga Presents, Kurumi recalls an incident with a young girl whose parents would give stuffed rabbit toys as a substitute for love. Kurumi takes pity on the girl and, even though she knows better, lies and says each rabbit is stuffed with her parents' love. The next morning, however, they are anything but loving, and the housekeeper tells the girl the only reason her mother had her was to replace a pet rabbit that died. The girl violently snaps, and later that night she cuts open her parents with a pair of hedge trimmers before tearing the stuffing out of the rabbit toys and stuffing into their dead bodies. It's implied she does the same thing to the housekeeper to "share" her parents love.
In Magic Knight Rayearth, being unable to save Princess Emeraude is considered this for Hikaru, Umi and Fuu. Mostly because they powered through the entire adventure, thinking it was completely straightforward and never thought twice about questioning their mission. It's so bad that, in the anime, Hikaru accidentally creates her own Evil Twin.
In the Area 88 manga, Hoover Kippenburg blames himself for the accidental deaths of several pilots during a botched training exercise back in Europe.
In Black Jack, a dying Dr. Honma contacts Black Jack to confess a shameful secret: when he first operated on Black Jack, he accidentally left a scalpel inside of him, but refused to believe it until he found the blade encased in a calcium shell during a follow-up operation years later. The mistake disgusted him so much that he found himself no longer able to trust his own skill as a surgeon, and retired from practice shortly after.
Spider Man didn't stop the burglar that would shoot his Uncle Ben (in various tellings, because of spite, laziness, or arrogance); later, he was unable to prevent his girlfriend Gwen Stacy from being killed by the Green Goblin. The latter is not helped by how his using his webbing in an attempt to save her caused her neck to snap from the recoil, a fact Marvel initially attempted to gloss over but has recently admitted (through, among others, What If? - where he saves her by diving in after her instead - and Spider-Girl).
Speaking of Spider-Man, for his foe J. Jonah Jameson, it was definately the Scorpion. Jonah paid private eye MacDonald Gargan to undergo an experiment that would make him strong enough to subdue and defeat the hero. The scientist conducting the experiment warned both of them that he "didn't know how it will affect your brain" but they went ahead with it anyway; true to the scienist's fears, Gargan was driven insane, becoming more of a threat than Spider-Man could ever become. Jonah has never even tried to blame this on Spider-Man (at least in any convincing way); he has taken full responsibility for the villain, and to this day, pays for Gargan's psychiatric treatment out of his own pocket and orchestrates compensations funds to aid victims of the villain's crimes. And to make it worse for Jonah, Gargan hates him even more than he hates Spider-Man for what happened to him.
Batman blamed himself for the death of sidekick Jason Todd aka the second Robin. In the animated Batman: Under the Red Hood, he and Red Hood both specifically use the phrase "my/your greatest failure" in reference to Jason's death. Some versions of Batman's story have him blaming himself for the death of his parents. (Notably, in Batman Begins, Bruce blames himself for the death of his parents, as they encountered Joe Chill after leaving the opera because Bruce was frightened by the bat-demon characters.)
That's not the only form of blaming himself for his parents - in some comics versions, he convinced his mother to wear pearls to the show, and those pearls are the reason they get robbed and shot.
Also in that movie, when Ra's Al Ghul tells Bruce that it's his dad's fault, he doesn't take it well.
He also blames himself for the creation of the Joker, since he knocked him into the acid; the torture of Stephanie Brown, because he lost control of Gotham during War Games; and the death of Ted Kord by Max Lord who hijacked Batman's creation, Brother Eye, to find Ted.
In some continuities, at least, there's also Harvey Dent, who Batman saw as a friend and ally that could really clean up Gotham in ways he himself could not - until Harvey became Two-Face.
For a time, he considered making Jean-Paul Valley Batman this, saying that it was done at a moment of weakness and it was a mistake.
At various points in his history, Superman has had the bottled city of Kandor to deal with. More recently, he's had the only time he's ever had to kill someone; this caused him to actually leave Earth for a while.
Now that one's only sorta in continuity. Hell remembers it enough to have one (1) demon capable of trying to corrupt Superman, but the man himself doesn't remember it.
Nowadays, Superman also feeling really guilty of being unable to help his childhood friend, Mon-El, conquer his deadly weakness for lead and leave the Phantom Zone where he was cast into at his request as the only way to save his life.
During the Silver Age and the Bronze Age, Superman's biggest failure was arguably when he destroyed Lex Luthor's protoplasmic lifeform that he created as well as causing his hair to fall out when he was trying to rescue Luthor from a lab fire when they were teens. Sure it was an accident and mostly not his fault (Luthor caused the fire and Superboy had no way of knowing what was inside at the time), but it didn't help that before the fire, Superboy jokingly said he could spy on Luthor to find out what he was working on. Luthor believed Superboy destroyed his experiment out of jealousy and dedicated his life to destroying him and proving he was better. Luthor had the scientific genius to make a cure for Kryptonite as well as unshrink Kandor, so if things had gone differently, Superman would have had a lot less problems in his life and an ally against evil as well.
A painful one for Superman happened just after his resurrection. After the excitement and relief of him returning from the dead, he and Lois raced off to Paris for a getaway to catch up. While he was gone, the Toyman kidnapped a bunch of kids, including the son of co-worker Cat Grant, Adam. Adam attempted to lead the other kids away, but Toyman caught them and slew them all. Superman was so distraught over this, it lead to him vowing never to take another vacation again.
Another one was when one of Luthor's schemes leveled Metropolis to the ground. Lois had to snap him out of it, reminding him that this wasn't Coast City and Metropolis could be rebuilt. It was, thanks to Zatanna.
One of the third Flash's allies-turned-enemies, Zoom, acknowledged this in a twisted way. He felt the speedster required a tragic failure to be a great superhero — even if Zoom had to make one for him. He later made good on his promise by causing the Flash's wife to miscarry the couple's unborn twins, right in front of the hero's eyes. The twins were later restored to life by the miracle of Time Travel, but the Flash still blames himself for Zoom becoming a villain in the first place.
Hal Jordan, the Silver AgeGreen Lantern, was away in space when his home city of Coast City was destroyed. He went crazy and became a villain, Parallax, dedicated to undoing the destruction through gaining cosmic power, time travel, or just rebooting the universe itself. (It was later revealed that the great failure had let an ancient evil trapped in the Central Power Battery of the Green Lantern Corps get a foothold on his mind.)
Another Green Lantern, Kyle Rayner, had his mother die of an unknown fatal disease. Even when Kyle, with God-like power, tries to resurrect her, she asks to be allowed to die. During the "Sinestro Corps War" Arc Sinestro reveals to a captive Kyle that her death was the work of an sentient alien virus working for the Sinestro Corps, and was all part of a plan to make him vulnerable to possession by Parallax.
That was Kyle's second My Greatest Failure. The first came early in his career when his girlfriend Alex was killed and Stuffed into the Fridge. Up until then he thought having a super-powered ring would be fun and a lot of laughs, but when that happened, Kyle grew up pretty fast.
The villainous robot Ultron was supposed to be Henry Pym's greatest failure, but it never seemed to take; later, he was given much more depressing sorts of failures to worry about (such as abusing his wife, betraying his teammates, and repeatedly going insane), while Ultron eventually (for a time, anyway) became more of a ludicrous than ominous figure.
During his tenure on The Avengers, Kurt Busiek did a good job of finally making Ultron into a credible threat, up to and including having him wipe a country's population off the map. Pym's destroying the primary unit with a metal disruptor, and a subsequent character arc that involved quite literal split personalities, were supposed to finally put Pym's demons to rest and let him start growing as a character again without the existence of Ultron and his other failures to continue hindering him. Yeah... not so much. Later writers actively regressed his sanity, brought back Ultron more than once, and had him replaced by an alien who tried to kill his ex before Thor accidentally turned her into a dimension, which Pym is obsessively trying to fix at the cost of his relationships with his team and Jocasta. As of the most recent Avengers stories, he's possibly even more screwed up now than he was then.
As revealed after Jan's death in Secret Invasion, Hank considers the worst thing he's ever done to be hitting his wife—and he admits that, given everything else that's happened, that's saying something.
Blue Devil's greatest failure was making a deal with Neron (DC's ruler of Hell at the time) to gain fame as an actor in exchange for destroying an electrical substation in the desert. This resulted in the death of his best friend Marlene Bloom, whose helicopter crashed because of the blackout. This would come back to bite Blue Devil again years later when Marlene's nephew and Blue Devil's sidekick Eddie Bloomberg/Kid Devil found out after making a deal with Neron for powers. This resulted in Eddie losing his trust in Blue Devil and Eddie eventually losing his soul to Neron once he turned twenty.
John Constantine, Hellblazer, botched a summoning and caused a little girl to be dragged down into Hell. This hung over him for years, until he was able, thanks to some clever manipulation and trickery, to free her soul and the soul of every other child in Hell.
In the Marvel Universe, Captain America originally was plagued with guilt about his sidekick, Bucky, being killed when he could have spared him by not allowing him to be his sidekick. However, the writers realized how old that got and had Cap's later protégé, Rick Jones, demand he get over it and move on. Furthermore, that guilt was replaced by the writers with Cap troubled by the state of the nation, which is at least more sophisticated and flexible a concern to use. Having said that, Bucky's death still influenced his interaction with younger superheroes, notably Spider Man.
His guilt came back when it turned out that Bucky survived, only to have been found by the Soviet Union and turned into the Winter Soldier; an elite assassin responsible for multiple murders. Cap considered this a fate worse then death and did his best to break his mental conditioning. Even afterwards he felt responsible for Bucky's well-being and put in his will that Tony should do his best to save him after he died.
In Top 10, Jeff Smax never really got over his failure to save a little princess from a dragon. Her handprint was permanently burned onto his chest, and would serve as a constant reminder of how big a screw up he was. Immediately after it happened, he informed the queen, and ran away. Across dimensions. To a world nothing at all like the one he came from. Even changing his name. Only after a necessary return to his homeworld, where the dragon was slayed and the handprint was erased, could he let it go, and only a little.
Scrooge McDuck once hired a group of thugs to case away an African tribe and burn down their village in an attempt to claim their land. This was the only time he ever made money dishonestly, and the resulting guilt (and zombie — as in Bombie the Zombie) has haunted him ever since. He even became a depressed shut-in for the better part of twenty years until his nephews were able to re-ignite his spark, leading to the adventures we all know.
In the Sonic the Hedgehog comic, it's recently been revealed that Monkey Khan was taken control of by the Iron Queen and forced to eliminate the Freedom Fighters in the Dragon Kingdom. Understandably, he harbors massive guilt toward himself and hatred for the Iron Queen, to the point where he was angry at Knothole for holding a celebration when she had taken control of the Eggman Empire.
Robotnik caused a lot of this. He caused the accident that resulted in Nate Morgan being exiled from the Overlanders' city, and years later, was saved from the overlanders by Jules and Sir Charles the Hedgehogs. He also banished Kodos to the Zone of Silence, sabotaged Sir Charles' roboticizer(Resulting in Jules becoming a robot), took over Mobotropolis from King Acorn...
In the Fantastic Four, Reed Richards' own miscalculation and arrogance are what led to the titular team being hit by gamma rays and given superpowers. In the case of Reed, Sue, and Johnny, this is no big deal. But Ben's powers leave him a freakish rock monster, and being trapped like that put a huge barrier between the two of them for years, in spite of being best friends. It wouldn't have been so bad if Ben could just change back from being The Thing, but Status Quo Is God so any changes back to a human are doomed to be short-lived. The fact that Reed is responsible for his friend's condition and cannot permanently cure him is something that has weighed heavily upon him his entire career.
Disproportionate Retribution indeed. In the original telling at least (Lord knows how many retcons may have flip-flopped since then) Doom didn't create the mask to hide a hideously mauled hamburger-face; he did it because the explosion left a thin, barely noticeable mar on his supposedly "perfect" facade. A man that vain, you could almost understand making some sort of covering... unfortunately in his haste, he didn't wait for the mask to cool before putting it on, which (Depending on the Writer) either DID hideously scar his face, or permanently bonded the mask to his skin. He might not admit the accident itself, but he DOES (very rarely) admit to the fact that putting the mask on so fast wasn't his smartest idea.
Actually all this was based on John Byrne's retcon of the Lee/Kirby origin, based on an idea by Kirby that either Stan Lee had discarded or Jack Kirby only had it after the original version was published in Fantastic Four Annual #2 (1964). There, Doom's head is completely bound up and hidden beneath bandages and in the caption Stan Lee unequivocally states: "As for Victor Von Doom... his face was hopelessly disfigured!" Also on the page after that, even though the mask is not completely cooled when it is put on Doom's face, it is cool enough for the monk to hold it in his bare hands (you can easily discern his fingernails etc.) By the evidence of that Annual, all the mask likely did was singe Victor's eyebrows.
In The Sandman, the immortal Hob Gadling will never forgive himself for having made his fortune in the slave trade.
Nightwing had a moment revealed following Zero Hour. When he was Robin, it was up to him to rescue Batman and the DA that replaced Harvey Dent when he became Two-Face. When he confronted the aforementioned villain, he was planning to hang the both of them for their "crimes". Two-Face attempted to hang the DA and Robin responded by cutting the line with a Batarang. However, he didn't realize that Two-Face's MO applied to everything - he saved the DA from hanging, but couldn't save him from drowning. Batman freed himself and saved the day, but the event haunted Dick until the first time he took up the Mantle of the Bat, finally making the save when confronting Two-Face again.
Nightwing also feels guilt over not only the crippling of Barbara Gordon, but the death of Jason Todd. Both of them stemming from the fact that at the time both events happened Dick and Bruce had a falling out which resulted in Dick leaving Gotham and establishing himself as Nightwing. He has notably taken a far more proactive role in mentoring subsequent members of the Bat-family (as well as serving as leader of the Outsiders and a key ally to the Birds Of Prey).
Rogue for a long time lived in deep guilt and remorse over what she had done to Carol Danvers. She was able to finally work past it, especially after Ms. Marvel was reintegrated with the stolen thoughts and memories through Applied Phlebotinum.
In Star Trek, Spock-prime's greatest failure was not arriving in time to save the planet Romulus from a supernova blast. This results in a mad man from the future seeking vengeance on The Federation and destroying Spock's home planet, Vulcan.
In Die Hard, Al Powell admits in the movie that he can't shoot a gun anymore since he shot a kid with a plastic gun. By the end of the movie, however, Al ends up killing The Dragon with a gun.
In The Killer, Ah Jong/Jeffrey Chow felt this way after he nearly shoots out Jenny's eyes at the nightclub at the beginning of the movie and towards the child whom he bought to a hospital after a shootout on the beach.
In Alien³, this is how Clemens got to Fury 161. He was previously a doctor, but he got addicted to morphine in medical school. When an industrial accident caused the deaths of a lot of workers, he was called in. Eleven people died not because of their wounds, but because he was so out of the world that he prescribed the wrong dosage of painkillers. He was jailed for seven years on Fiorina, and his medical license was reduced to a Class C. When the facility was due to be closed down, he elected to stay on as the medical officer when the inmates didn't want to leave.
In 42, this is the real reason why Branch Rickey tirelessly fights for integrating Jackie Robinson into the Brooklyn Dodgers. Rickey's lifelong love of baseball was spoiled when he was unable to recruit a talented African-American player due to segregation.
Jake from Animorphs, whenever he recalls the David incident. Also the fact that Jake couldn't save Tobias from being trapped as a hawk.
Even moreso, the fact that he couldn't save Rachel or Tom from dying, not to mention the order to kill seventeen thousand Yeerks. Those two failures become to the entire focus of his personality for about five years.
In the Redwall novel Martin the Warrior, when the title character's girlfriend is killed fighting alongside him in battle he blames himself and goes into self-imposed exile, setting up the events of Mossflower to which that book was a prequel.
Prophet's House has Sir Magnus, who lost a critical battle during a war between his patron House and its enemies. He's also The Atoner.
John in The Grapes Of Wrath thought his wife had a stomach ache when what she really had was much more serious. He's never forgiven himself, and his every action is driven by a desire to atone.
Looking for Alaska's titular character blames herself for her mother's death when she was a little kid, as she was too shocked to call an ambulance. Later in the novel, most of the principle cast (including the narrator) gets their own greatest failure when Alaska dies in a drunken car accident halfway through the novel. The kicker? The reason she was driving in the first place was because she realized she forgot it was the day her mother died and was trying to visit her mother's grave.
In Harry Potter and the Deathly Hallows, it's revealed that Dumbledore is wracked with guilt at his failure to take care of his little sister, whom both of his parents died to protect. He instead chooses to run around with Grindelwald espousing anti-Muggle ideals; and she is accidentally killed during a fight they have. He learns from the whole thing that he ought not to be trusted with power; and it certainly seems to be his one failure.
It could be argued that Harry's own greatest failure comes in Order of the Phoenix, when his overconfidence and disregard of Kreacher ultimately result in Sirius' death.
Sirius' greatest failure: convincing James to let Pettigrew become Secret Keeper, not knowing that Pettigrew was a Death Eater.
Sniper-turned-pilot Myn Donos led the shiny new Talon Squadron into a trap, and only he and his astromech survived, through sheer luck. He's the poster child for Heroic BSOD and Defrosting IceSniper, blaming himself. Interestingly, while he isn't explicitly blamed by others, his instructor wonders if he is such a bad teacher that he can't teach squadrons the quick thinking and flexibility it takes to survive an ambush.
Donos was inducted into Wraith Squadron, known for being populated by people on their Last-Second Chance. Another Wraith, Castin Donn, was a slicer in a Rebel cell on Coruscant. When they received the broadcast of the second Death Star exploding and the Emperor dying, he hacked public viewscreens to display it, since there was no way the citizens would see it without the filter of Imperial propaganda otherwise. Crowds went nuts, riots and wild celebrations ran through the streets, and in one plaza a huge statue of the Emperor was torn down... and then the stormtroopers came and fired into the crowd, killing many of them. Castin holds himself at least partially responsible.
Dia Passik faces her greatest failure in Iron Fist, where she is forced to shoot Castin during a botched infiltration job. (He was probably already dead.) She comes out of the experience both disgusted by her seeming failure and frightened of her own ruthlessness. However, her companions and superiors praise her actions, telling her that by doing what she had to do, she saved the rest of the team and their whole operation.
Kell Tainer has his problems with anxiety and cowardice during missions in Wraith Squadron, which he considers his own major failing, although he eventually deals with them. Given how much of a Dysfunction Junction the Wraiths are, it's not surprising these are handed out pretty liberally.
Sherlock Holmes often dwells on his defeat at the hands of Irene Adler in A Scandal In Bohemia, although he holds no real grudge against her. Ironically, the tale of one of Holmes' greatest failures is what first popularized him. While the first Holmes story was A Study in Scarlet, the character and his world didn't hit the big time until A Scandal in Bohemia proved to be a runaway success.
As quoted above, The Yellow Face is a case where his theory about the cause of certain mysterious events proves to be wrong. Yes, even Sherlock Holmes makes mistakes. *
Despite being the page quote, this case does not actually apply, as the situation turned out excellently well for all involved and the worst harm was to Holmes' pride, since the actual scenario was much Lighter and Fluffier and featured much less betrayal than he had thought. Do Not Multiply Entities Beyond Necessity!
Agatha Christie did an hilarious subversion of the trope, and a very obvious Take That against the original Sherlock Holmes example just above. Hercule Poirot, the brilliant if egotist detective, retells, at Hasting's insistence, the story of his only failed case, which had involved a chocolate box. He then tells him to whisper "chocolate box" to him whenever he gets too pompous, because he tends "to get over-confident thanks to his super-human intelligence." Hastings then immediately says "chocolate box." Poirot doesn't get the joke. At all.
Grand Admiral Thrawn of The Thrawn Trilogy pulls off his tactical genius through the psychological insights into alien enemies he gains from studying their art. Just once, he failed to gain any insight - and he keeps the original piece of art to remind him. By the time of the trilogy he thinks he's finally starting to understand... not that it will be any help in the future. He'd had to destroy the planet. Pity.
Navidson's greatest failure in Houseof Leaves was his failure to save Delial, a little African girl dying of starvation, and taking her picture instead. This would continue to haunt him for years.
Jemidon from Secret of the Sixth Magic by Lyndon Hardy is haunted by the memory of his dead little sister, who wouldn't have died if the gold coin his parents gave him to pay for his test as a would-be thaumatuge had been spent on medicine instead. He failed the test, and both he and his parents blame him for her death ... which is insane, because he was only ten when his folks urged him to get tested, and they're the ones who'd set ambition for one child above the life of another.
In The Dresden Files, Harry considers his lover's half-conversion to a Red Court vampire and his inability to find a cure for her to be his greatest failure, driving him to near-poverty and a long Heroic BSOD shortly after it first happens. Years later, when he destroys the Red Court at the cost of said lover's life and then learns that that act cured all the other half-turned people, he says that whatever number of people he saved, it will always be one too few.
Sisterhood series by Fern Michaels: Played with rather strangely with Henry "Hank" Jellicoe. Game Over indicates that there is one topic that he refuses to discuss, to the point that it is not even stated what the topic is. Deja Vu reveals that the topic is his wife Louise. She and her daughter left him and went under Witness Protection a long time ago, and he, with all his power, has never been able to find her. However, he had treated her like she didn't exist and was just a servant. He took phone calls on his illegal dealings, and he did it right in front of her! She kept a diary of his dealings that apparently ended up in the hands of the CIA, and he, with all his knowledge, has never been able to confirm the story. He wants to find her... and then kill her for having the nerve to go against him and leave him! Even villains can have a My Greatest Failure.
In Anne McCaffery's Damia, the title character accidentally fried the mind of her first lover. She considers it this because not only did she never consider keeping her mental guard up while with a far lower T-rating, she ignored Afra's warning to "be careful" out of spite, due to a fight they'd had earlier (After Afra had rebuffed a rather unsubtle attempt at seduction). This helped drive a wedge between herself and Afra that lasted a decade*
Though not only did Afra actually want her, but wanted Damia to see him as himself and not just "Good Ol' family friend Afra", but she thought the whole thing was being driven by her mother,The Rowan.
Later books showed that Damia made sure her children knew all about "the facts of life" so they wouldn't go through what she did.
Bartimaeus feels the death of his master Ptolemy in order for Bartimaeus to live is this, and his guilt is so bad he wears the face of Ptolemy millennia after the boy's murder.
Bartimaeus: "It's two thousand, one hundred and twenty nine years since Ptolemy died. He was fourteen. Eight world empires have risen up and fallen away since that day, and I still carry his face."
Beachwalker's protagonist had one of these in the form of her mother's death. She is determined to keep the past from repeating itself, whatever the cost to herself.
In the Ranger's Apprentice book The Lost Stories, it's revealed that Halt's greatest failure is accidentally causing Will's mother's death. Thankfully, nobody blames him for it.
Twenty years before the events of The Sharing Knife books, as he faced the creatures of a powerful Malice during what became known as the Battle of Wolf Ridge; Patrol Leader Dag Wolverine lost all but three of his joint command, his left hand, his wife, and the ridge in question in the space of an hournote (the successful delaying action aspect that facilitated the death of the Malice in question was cold comfort). Heroic Songs and Epic Poems have been composed since, and Patroller Dag Redwing tends to slip out the back when younger generation of Lakewalkers start in on them.
Forever Gate: Hoodwink never forgave himself for allowing a rich guy to brainwash his daughter into the ideal House Wife. It lead him to drinking which drove away his wife. Now he throws himself into harm way's way whenever Ari's in danger to make up for it.
Live Action TV
Wesley Wyndham-Pryce from Angel committed his greatest failure when he falsely abducted Angel's only child in order to save both from demise. The prophecy which led Wes to believe this, was altered however, never meant to come true.
Charles Gunn's greatest failure was when he made a deal with Dr. Sparrow to make his legal upgrade permanent in exchange for signing to release an ancient curio stuck in customs. This results in the death of Fred and resurrection of the demon Illyria. Gunn becomes so guilt-ridden, that he offers to take Lindsey's place in a hell dimension to get information to stop the Senior Partners.
In between the original and new Doctor Who series, the Time Lords and Daleks fought a war that annihilated both sides. The Doctor's greatest failure is letting his people die and survivinghimself.
It is revealed in The End Of Time, the final 10th Doctor episode, that the Doctor really had no choice whatsoever. The war had brought out the worst in the Time Lords and had made them into Omnicidal Maniacs as bad as the Daleks, and who were going to destroy all of time and space while cheating death by becoming beings of pure consciousness. So the Doctor chose to lock them and the Daleks and everything else involved in the war outside of normal time and space where they would annihilate themselves while keeping the universe safe.
Before that, The Doctor felt great guilt over not having the guts to assassinate Davros, creator of the Daleks when he had the chance.
Adric's death. Really, the loss of any companion wounds the new Doctors deeply, and has them beating themselves up for a good while afterwards.
He's also convinced that he ruins his companions' lives. When the TARDIS voice interface takes his form in Let's Kill Hitler, he tells it to "give me someone I like." Holograms of Rose, Martha, and Donna are met with a response of "guilt", "also guilt", and "more guilt". He finally settles on the image of little Amelia, "before everything went wrong."
In Star Trek: The Original Series, Captain Kirk failed to restrain his friend Commander Mitchell when he began exhibiting God-like powers, although he was warned by Mr. Spock. It wasn't until Mitchell killed a crewman and set about destroying the Enterprise that Kirk took action.
Likewise in Star Trek II: The Wrath of Khan, Kirk's blunder in failing to arm the Enterprise in time resulted in Khan blasting the ship nearly to hell. He came back nicely though....
Both of these pale in comparison to Kirk's real greatest failure: he failed to save his son, David.
And prior to that, he was haunted for over a decade by the deaths of over 200 crewmen (including his CO) because he thought his hesitation in firing on the Monster of the Week allowed it to attack. It was only when he saw it would have made no difference did he accept his choice.
Subverted in the Star Trek: The Next Generation episode "Tapestry", where what Picard thought was his worst failure was picking a fight as a cadet and getting an artificial heart due to getting his real one stabbed. As Q showed, that incident made Picard what he is.
Played straight in Star Trek: First Contact, where Picard's real greatest failure was being unable to stop the Borg from using him as part of their invasion in "The Best of Both Worlds".
In Star Trek: Voyager, Chakotay failed to catch (and ended up falling in love with) the Cardassian spy Seska in his Maquis cell, which came back to haunt him when she betrayed the series' titular ship and its crew to the evil Kazon. To add insult to injury, he also failed to catch the Federation spy Tuvok, compounding the guilt over Seska's betrayal. He only (sorta) recovered when he realized that neither spy realized the other was in the cell, making the incident not (entirely) his fault.
Chakotay (to Tuvok): "She was working for them, you were working for her... was anyone on my ship working for me???"
One of the defining moments for Firefly's lead character, Malcolm Reynolds, was the complete, total, and utter defeat he suffered at the Battle of Serenity Valley. By the time the battle was over, he had lost his faith and had been turned from a chipper, patriotic, and energetic soldier into the tired, cynical bastard he is in the series.
Delenn's greatest self-recognized failure was ordering the complete annihilation of the entire human race over an accident that killed her teacher. Despite what they claim, Minbari are extremely compulsive and bloothirsty.
Londo Molari had two greatest failures:
Giving a family heirloom to his mistress (the only person he ever really loved) to wear, making her an easy target for Morden to find and murder.
Getting involved with Morden and his "associates" in the first place, thus kick-starting the entire Shadow war. Londo's life was one long string of incredibly bad choices, motivated by self-interest and fear. By the time he discovered he was on the wrong path and began walking the right path, it was far too late for him to save himself.
G'kar had at least one greatest failure: Torturing Londo and submitting him to literal Mind Rape to discover how completely Londo fistfucked the Narn people with the Shadows' help. It took Kosh's intervention to show G'kar that he would eventually fall as low as Londo had, before setting G'kar back on the path to redemption.
Stargate Atlantis. Rodney Mc Kay's unintended destruction of a solar system (well, five sixths) is brought up occasionally by Sheppard, though usually as a joke, seeing as the system was uninhabited.
Stargate SG-1: Lt. Col. Cameron Mitchell's is when he bombed a truck he thought was carrying enemy soldiers, but was instead carrying civilians.
Jack O'Neill's is the fact that his son shot himself with Jack's gun.
Torchwood. Jack's inability to find (or redeem) Grey.
Jack. "Day Four" with Ianto's death. This, in combination with his decision in "Day Five" to save the day by killing his grandson (although this doesn't count as a failure, despite how tragic it was) was enough to convince him that he had to completely leave the Earth, despite Gwen's best efforts.
In Homicide: Life on the Street, Detective Bayliss is haunted by his failure to solve his very first case, the murder of a little girl called Adena Watson. Although he makes numerous efforts to put it behind him, and later develops from the rookie he was when he caught the case to a competent, seasoned homicide investigator, his obsession persists throughout the entire series, even after it seems that the girl's family have moved on.
The case is based on the real life unsolved murder of Latonya Kim Wallace, the "Angel of Reservoir Hill", the investigation into which is covered in the book upon which the series is based. As noted in the book, the real life detective in charge of the investigation grew fixated on the case, but the epilogue stated that at time of writing he was beginning to accept it and move on.
Detective Monk has solved every case he has come across. However, he's been stuck for a long time on that little case of his wife Trudy being blown up by a car bomb.
In Smallville, Clark Kent has had several, including when he reversed time (a one-use only deal) to save Lana's life, only for his dad to suffer a heart attack and die at the end of the day. Also, John Corben/Metallo blames him for his turn to evil, because when Clark saved a bus from crashing, a passenger from that bus went on and murdered Corben's sister the next day. But in his opinion, his greatest failure is probably in the season eight finale when a complex chain of actions lead to the death of Jimmy.
24: Jack's failure to protect Teri from the inherent dangers of his profession.
In Mighty Morphin' Power Rangers, failing to retrieve the Green Candle in season one, thus saving Tommy from losing his powers, was shown to have haunted Jason for a long time, to the point where he even makes reference to it in Zeo.
Andros.Dear Lord, Andros. First he loses his sister as a child and she grows up to become one of the Big Bads serving the Bigger Bad, then his best friend Zhane is nearly killed during a battle and Andros puts him in cryogenic stasis to keep him alive. They both get better.
Shotaro Hidari blames himself for the death of his boss, Sokichi Narumi, primarily because he ignored Sokichi's orders on a particularly dangerous job, which lead to guards chasing them and gunning Sokichi down. As a result of this and Sokichi's Famous Last Words, Shotaro lives his life trying to achieve the "hard-boiled detective" ideal his boss embodied.
In The X-Files, Mulder is plagued by guilt over not protecting his younger sister, Samantha, from abduction when they were children, despite the fact that it involved circumstances well beyond his control. His parents don't help assuage his guilt over it; at one point a clone claiming to be his sister is killed, and his father is upset and tells Mulder that he has to tell his mother that he lost his sister... again. As a result, he becomes obsessed with protecting Scully, especially after her abduction in season two. He ditches her several times, to her annoyance, and at times demands she stay out of a case for fear of her life. And woe is you if you're the one who hurts Scully.
In Warehouse 13, Myka is wracked with guilt over the death of her partner and lover, while she was in charge of the mission. For Pete, it's the death of his firefighter dad in the line of duty, when Pete decided not to tell him that he had one of his bad feelings. One episode serves to get both characters to come to terms with their respective guilts and realize they're not at fault. Myka's partner disobeyed her orders and got himself killed, while Pete's dad would've done his job no matter what his son said. Everything H.G. Wells does is because of the death of her daughter during a home invasion a century ago, while she was out of town.
Arturo from Sliders considered his work on traveling to parallel universes his greatest failure until Quinn Mallory discovered how to do it.
The title character of Merlin is an Iron Woobie turned Up to Eleven. But when he fails to heal the king and quite possibly turns Arthur away from magic forever, he very nearly crosses the Despair Event Horizon, and only stops because he's reminded that Arthur will need his support as king.
Rumpelstiltskin in Once Upon a Time only broke one promise, the one he made with his son Baelfire. He was given the chance to live with his son in another world without magic to free him from the power of the Dark One, but was unable to let go of his power and lost his son as a result. He set out making the Dark Curse so he could find a way to fix this.
Jiminy Cricket saw what happened to Geppetto's parents as this for him. He sets himself as The Atoner to repent.
Go Namsoon he breaks his former best friend Heungsoo's leg, while him and the the rest of his gang were beating Heungsoo up as part of the process and punishment for him leaving. Heungsoo had been told by his coach that if he had serious aspirations of playing football professionally he couldn't be part of the gang any longer, but this injury meant he could no longer play. After this Namsoon experiences Heroic BSOD and spends a year eating, sleeping and crying.
Kang Sechan His student commits suicide because he doesn't set aside the time to talk to her when he comes to him. This causes him to give up teaching in school for fear of failing another student.
In the UK version of The Voice, Will.i.am. (one of the coaches for the UK series) called himself an idiot for not hitting his button during seventeen-year-old self-trained opera prodigy Shansel Husayin's note-perfect performance of "Nessum Dorma", and profusely apologized to her for not doing so, stating that he realized too late that she was an opportunity to "reinvent pop radio." As late as a mid-2013 interview, he's apparently still kicking himself in the butt for letting her slip through his fingers.
For decades in Forgotten Realms, Drizzt Do'Urden had kept a vow to never kill another dark elf. However, he couldn't keep it forever, and in order to escape, had to kill one of his kin. He was guilt-wracked for this... but, in a possible subversion, not for long, as he realized it was a hypocritical vow, given that he had often seen the necessity of killing orcs, humans, goblins, duergar, wererats, and others who actually tended to be less Always Chaotic Evil than his own people.
Another 40K example is Sarpedon of the Soul Drinkers, who nearly led his Chapter into the clutches of the Dark Gods... and has eight legs to remind him of the danger of Chaos.
The Emperor is not dead. He is the living God of the Imperium who sits upon the Golden Thrones and guides us through these trying times. To say otherwise is Heresy. Repent immediately, before I have a chance to get the Promethium.
In Exalted, this is how Green Sun Princes become Green Sun Princes. They're on the edge of a heroic, epic act that would grant them Solar Exaltation... but then they choke, and then, in their moment of weakness, despair, and regret, the Yozis are there to offer them the Deal with the Devil that grants them the Infernal Exaltation.
Lesovikk in BIONICLE once briefly hesitated in a fight against a group of Zyglak, costing him his entire Toa Team, and driving him into a millennia spanning Heroic BSOD and Walking the Earth for ages.
In Final Fantasy VII, Cloud feels regret over several failures. His failure to become a SOLDIER, the death of Zack who died trying to protect him, and blaming himself for not being strong enough to protect his childhood friend, Tifa, when she falls into a ravine and slips into a coma. It also didn't help that Tifa's father blamed Cloud for Tifa's injury. leads to Cloud suffering from psychological disorders like Trauma Induced Amnesia and Split Personality. He also regrets failing to prevent the death of Aerith, which motivates Cloud to take down Sephiroth.
In the prequel Crisis Core, Zack's failure in capturing Genesis and preventing the death of Angeal also fall into this category.
In the backstory for Halo: Reach: both Carter and Kat consider the same event, the death of their squadmate Thom, to be Their Greatest Failure. Kat planned the operation he was killed on, while Carter blames his "inadequate team preparation." However, to quote their commanding officer:
"Eventually I hope to be able to get it through their thick Spartan skulls that Thom is dead because he chose to pursue a group of enemy combatants ON HIS OWN rather than wait for backup."
The Didact seems to consider his life to be this trope. Particularly his hand in nearly wiping out humanity over a misunderstanding, losing every one of his children in the effort, being defeated by the Builders in his effort to prevent and safeguard against the inevitable return of the Flood, and, to top it all off, he dies without completing the potentially galaxy-saving mission he was sent on by his wife. Due to his reincarnation in Bornstellar, though, he lives just long enough to fail to save his beloved wife and galaxy from the Flood and is forced to commit galactic genocide via the same drastic methods he fought against in his aforementioned failure with the Builders. This guy just couldn't catch a break (except for that thousand-year coma he was forced to go into after the again-aforementioned struggle against the Builders).
Godot/Diego Armando projects this onto Phoenix Wright in Ace Attorney: Trials and Tribulations, feeling that the death of Mia is Phoenix's greatest failure. It's a cover for blaming himself.
Detective Badd of Investigations wears a bullet-riddled trenchcoat to remind himself of his failure to protect Cece Yew from being murdered by a smuggling ring before she could testify. This also led him to become part of the Yatagarasu.
In Tales Of Symphonia, Lloyd and Genis are most troubled by the mutation and death of their friend Marble after Lloyd had been caught sneaking into the ranch to visit her with Genis, which also led to an attack on Iselia. It's made worse when her granddaughter Chocolat refuses to be rescued after learning this.
Sheena is also burdened by her failure to make a pact with Volt, even though she was unable to understand what he was saying and (unbeknownst to her at the time), he no longer wanted to make a pact.
Kratos has at least two of these, both spoilery: his inability to prevent Mithos from falling into despair after Martel's death, and being forced to kill his wife after she was turned into a monster. No wonder he's a Death Seeker.
In Metroid: Other M, Samus's falling out with Adam is implied to be a result of her greatest failure. The incident in question turns out to be trying to get Adam to change his mind about leaving his brother Ian on a ship that is about to explode, making an already hard decision even harder.
Pick a BioWare game, and chances are you'll hear one of these from at least one of your party members. Chances are, you'll end up with them when they attempt to correct said failures, too.
A more comprehensive list:
In Mass Effect, Garrus was rather angry about an organ farmer that C-Sec's regulations prevented him from stopping. His personal mission in that game is to hunt down and kill the organ farmer.
In Mass Effect 2, you recruit Garrus from the battlefield in which most of his team was slaughtered. His loyalty mission is to hunt down the man that sold them out.
Also in Mass Effect 2 is Thane's regret that he left his son behind following his vengeance on his wife's murderers. His loyalty mission is to keep Kolyat, the son in question, from following in his footsteps.
While Samara cannot be blamed for having daughters who are Ardat-Yakshi, Asari with a rare genetic disease that makes them addicted to murdering people by draining their life energy, she later makes it her life's mission to hunt down and destroy Morinth, the daughter who ran to preserve her freedom rather than join her sisters in isolation at a remote monastary where they learn to supress their urges and are far removed from any temptations.
In Mass Effect 3, being forced to leave Earth in the midst of a Reaper invasion is this for Shepard until the fall of Thessia where s/he hits his/her lowest point, blaming him/herself for not getting the right info and for the fall of the asari homeworld.
In the same game, if Shepard tries to convince Mordin to fake curing the genophage, Mordin will finally break down and confess that he views his works to negate the slow krogan development of immunity to it as this. He spent the previous game and his earlier interactions in this one denying that he felt any guilt over it, though.
In Jade Empire, Sagacious Zu regrets his failure to save Master Li's family from the Emperor's vengeance. You learn later that Master Li's daughter is still alive, and she happens to be right in your party.
In Dragon Age: Origins, Wynne was assigned to mentor a young elven mage during her younger days, and he ended up fleeing the Circle and getting killed by the Templars. In her personal quest you learn that he actually escaped and Wynne was told he had died to mess with her, and the two get to meet again.
Sten considers losing his sword to be his greatest failure. Swords are very important to the Qunari.
Oghren also blames himself for his entire clan dying and his wife Branka going insane, believing that had he been a better husband she may have not gone on her mad quest for the Anvil of the Void and dragged their clan into their gruesome fate.
In Knights of the Old Republic, Carth considers his failure to stop Admiral Karath from turning on the Republic to be his greatest failure; about three-quarters of the way through the game he gets to kill the Admiral.
In Neverwinter Nights, the confused angsty mess of a person that is Aribeth considers her fiancee's unjustified execution at the end of chapter 1 to be her failure.
In the ''Sam & Max'' game The City that Dares Not Sleep, Sam's greatest failure is not being able to rescue Max from "The Final Imperative;" Max was Killed Off for Real, and most of the game's ending scene follows Sam's guilt and grief over Max's death.
In Metal Gear Solid 4 Big Boss views the killing of his mentor and friend who taught him everything he ever knew as a soldier and then going on to fight for causes that she never would have believed in as his greatest failure. He goes so far as to claim after that point he was already dead emotionally. At the end of Metal Gear Solid 4 as he stands above her grave, Big Boss proclaims to his son that if their roles had been reversed, he probably wouldn't have made the same mistakes and that he still has a chance to do things better than he ever had.
In Okami, when Waka tried to save the Celestials using the Ark Of Yamato, he didn't realise that Yami was on board, leading to the Celestials getting slaughtered. It also lead to a horde of demons being unleashed in Nippon.
Goenji Shuuya (Axel Blaze) in Inazuma Eleven blames himself for his sister's serious accident a year before the game's event because playing a national soccer tournament against Teikoku lures bad guys into harming his family and put his sister into a coma, but is snapped out of a Wangst state by The Hero's Hot Bloodedness. Then he is determined to win a Football Frontier tournament for his sister.
In Persona 3 FES, one of the things The Answer deals with is Aigis's Greatest Failure. That is, her inability to prevent the Protagonist's death despite her promise to protect him.
In Tales Of The Abyss, Jade's backstory includes having accidentally killed his teacher with experimental magic as a child. He's mostly gotten past it now, but it drove him to extreme lengths trying to make a Replacement Goldfish for about ten years until Peony knocked some sense into him.
The magic he developed in this time - fomicry, used for making clones called "replicas." - is what kicks off the whole plot, allows the main character, a replica, to exist, and forms the basis for the Big Bad's plan. In a way, Tales Of The Abyss overall is Jade Curtiss' greatest failure. He is very aware of this.
Jade: I wish I could go back in time and kill myself as a newborn.
According to the original backstory, Serious Sam was the captain of the starship that drew Mental's attention to humanity and he threw himself headlong, even suicidally, into the fighting to try and atone.
Ethan Mars from Heavy Rain believes that his his son Jason's death was his fault, and seems to suffer some degree of PTSD as a result.
Azimuth:Because I failed them! I... I failed them...
Deckard Cain of the Diablo series has much to regret, as shown in his dialogue in Diablo III:
Deckard Cain: I am the last of the Horadrim. I couldn't always claim this. In fact, if I had turned to the Horadric teachings sooner, Diablo could have been stopped before his reign of terror began.
Player: You cannot believe this.
Deckard Cain: When the first signs appeared, I did nothing. I had shrugged the old tales off like they were so much fantasy, and Tristram — no, the world — has paid dearly for my arrogance.
Real-life example with Blizzard concerning their on-going development of World of Warcraft where they flatout stated during a Blizzcon interview that their biggest regret for the franchise was not creating Azjol-Nerub into a world zone during the Wrath of the Lich King era.
According to thisAngry Birds book by National Geographic, the Mighty Eagle's backstory was that he went into a self-imposed exile after letting the pigs steal the eggs under his watch. His only friends who visit him regularly are the Blues, who feed him cans of sardines and have him tell them stories.
In El Goonish Shive, Grace cowered as Damien went on a killing spree breaking her and her brothers out of the lab that created them. Grace later finds out that she was created to fight Damien, and her guilt over failing to act the first time overcomes her pacifistic tendencies and drives her berserk when Damien threatens to kill Nanase and Ellen. Despite saving her friends' lives and freeing her brothers by defeating Damien, she feels even more guilty for losing control and then failing to keep him from committing suicide. This hat trick of perceived failure leads her to swear off using her shapeshifting powers until her sister and grandfather convince her to stop blaming herself.
Vaarsuvius from The Order of the Stick is a highly intelligent elven wizard with a large ego. That ego takes a severe beating when the elf is unable to defeat a highly spell-resistant death knight, fails to prevent the hobgoblins from overrunning the breach at Azure City and winning the battle (including having fleeing allied soldiers beg him/her to help them and one of them cursing the elf's "useless goddamn magic" with her dying breath after V is unable to help them due to having run out of spells), and then spends months trying to overcome the effects of an anti-scrying abjuration without luck, in a situation when the fate of the world practically hinges on the elf's success. The long-term consequences for Vaarsuvius remain yet to be revealed, but in the short term, they certainly include Bad Dreams, every symptom in the book for Post Traumatic Stress Disorder, and easy temptation to a Face Heel Turn when his/her self-perceived lack of ability puts his family in danger.
Supplanted now by the realization that the Familicide spell (s)he used during said Face Heel Turn wound up being responsible for the death of a staggering number of innocent people.
Elan gets a bit of this when the half-orc Therkla is killed trying to protect him. She's poisoned, and Elan lacked the Bard spell that would have cured her. It affects Elan enough that when he reunites with Haley, he reveals that he took "Neutralize Poison" at his next level up.
Homestuck: Even after 413 years, WV still won't let himself forget that he failed to prevent Jack from murdering his army.
In Survival of the Fittest, Adam Dodd doesn't only have one of these, but two. The first of these is allowing himself to become separated from the other members of his group - among them his girlfriend and other close buddies of his. They all proceed to be killed, and in one case, raped. Adam blames himself for this. His second stems from an incident where his (mentally unstable) brother attacked him. Adam regrets throughout version 1 his failure to forgive his older brother until one of the very last scenes of the V1 endgame.
In Greek Ninja, Sasha fails to save her sensei during the invasion of Ariadnio.
In Teen Titans, Robin considers Red X to be this, not only because the plan didn't work, but because it caused him to lose the trust of the rest of the team, and he was lucky he got it back. This is made only worse when someone steals the equipment and becomes the villain again. Nothing can convince him that it isn't anyone's fault but his own (and he has a point).
In the third season of The Transformers, Optimus Prime's successor, Rodimus Prime (formerly Hot Rod) doubted his abilities to fill Optimus's shoes, in part because during Optimus's last stand, Hot Rod tried to join the battle and help, only to wind up being used as a hostage/shield by enemy leader Megatron—had Hot Rod not stepped in, Prime may have survived. Visiting Prime on his deathbed, Rodimus asked for forgiveness... But Prime slipped away before he could forgive Rodimus.
Although he did promote Rodimus to Prime through the Matrix, so that probably implies forgiveness.
Aang's loss to Azula in the second season finale, resulting in not only the loss of the entire Earth Kingdom but Aang's death as well. Cue Aang stealing Zuko's "I must restore my honor" speech when he gets better. He expresses a similar regret over running away, feeling that he's responsible for the escalation of the war and the destruction of the Air Nomads.
Aang's predecessor, Avatar Roku, had several opportunities to stop his best friend Fire Lord Sozin from starting the hundred-year war in the first place. Sozin expresses to Roku that maybe he should Take Over the World to share the "greatness" of the Fire nation? Roku, without even, you know, explaining to Sozin just why this would be a terrible idea, just blows him off and says forget it. Sozin sets up colonies in the Earth Kingdom? Roku fights him off but doesn't kill him, incarcerate him, warn the Earth Kingdom and the Water Tribes of his leanings, or anything beyond telling Sozin "never try this again or else". Is there any surprise at Sozin's dick move in letting him die from the poisonous gases of that magnetic eruption so he can be out of the way and Sozin can start his invasion? Naturally, in Roku's afterlife and the twilight of Sozin's life, both regret what happened.
A side-story, shown as motion comics, gives Avatar's Kyoshi and Kuruk one of these. Kyoshi formed the Dai Li, which went on to assist an Evil Chancellor from turning Ba Sing Se into a totalitarian city-state which carried after Azula conquered it. Kuruk wasn't a good Avatar, spending most of his time impressing people with his bending prowess. But when he found a wife and started shaping up, Koh the Face-Stealer snatched her, possibly to punish him.
For Uncle Iroh, it's his failed siege of Ba Sing Se during his time as a Fire Nation general. It's hinted many times that the failure isn't because of the military disgrace, but because he lost his son in the battle.
It wasn't so much as a failed siege, as he is the person credited with the breaching of Ba Sing Se's walls, then thought to be impenetrable, from the outside. He abandoned the siege due to the death of his son. His brother Ozai, still considers his abandonment of the siege the greatest failure though.
Transformers Prime: Arcee takes the deaths of her partners Tailgate and Cliffjumper very personally. Whenever either of them comes up in conversation, her normally assertive confident personality slips into rage, and the possibility of it happening again turns her into a nervous self-doubting train wreck of a person.
Ratchet greatly regrets not being able to save Bumblebee's voicebox even though he saved his life during the Great War.
Princess Celestia from My Little Pony: Friendship Is Magic seems to feel this way about being forced to imprison her own little sister, Luna, in the moon when Luna became Nightmare Moon. Luna, after being redeemed, seems to feel this about becoming Nightmare Moon in the first place. At one point of "Luna Eclipsed", she's seen looking forlornly at the statue of her evil self and has become The Atoner.
For Splinter in Teenage Mutant Ninja Turtles (2012), it was the act of allowing his temper to get the better of him when Oroku Saki insulted him in front of Tang Shen. The resulting attack intensified the rivalry between Oroku Saki and Hamato Yoshi (as Splinter was previously known as), eventually leading to Tang Shen's death, and Yoshi/Splinter's daughter being kidnapped. This is why when it comes to Raphael's temper, Splinter treats it as Serious Business.
Subverted in, of all places, Ultimate Spider-Man. A new villain Nightmare uses his powers to transport the team into dream worlds where he makes them confront their greatest fears. Spider-Man is forced to come face-to-face with his murdered Uncle Ben... and strikes up a pleasant conversation with him. Nightmare is shocked that Peter isn't grieving and Peter tells him why it's not working. Yes, Peter regrets what happened to Uncle Ben, but he turned that tragedy into something good by becoming Spider-Man, a hero. So if he expects a guilt trip to keep him from kicking his butt, he's got the wrong person.