Tragic Mistake

Tragic Mistake is the event where a hero is given a clear choice, but because they choose poorly, it results the disastrous event that brought the huge impact on their life that made them more miserable than ever. The said mistake may caused by the said hero's own flaws, but sometimes fate also pressed them to chooses poorly as well. While the hero and/or their allies may not realizing the consequence of their wrong choice, that moment is crucial to the beginning of unfortunate event that ranges from merely the hero's tragic downfall to the chain of disastrous events.

The results of this bad choice lead inexorably towards the hero's catastrophic end—had the hero chosen correctly at this point, the catastrophe could have been averted. The said mistake also the crucial part of their story.

Star Wars

 * Anakin Skywalker had made many mistakes prior to this point, but what pushed him irrevocably over to the Dark Side was his decision to save Emperor Palpatine, leading directly to Mace Windu's death and leaving him no chance to go back as well downfall of Jedi Order.
 * Another point of no return for Anakin was when he impulsively (and unnecessarily) refuses to see Padmé Amidala, trying to talk him down and Obi-Wan Kenobi stowing away on her ship, as anything but an attack on him. This results in him Force choking his wife and dueling with his former Jedi Master, ending with him having his three remaining limbs chopped off and getting burned very badly by the lava. The old Anakin might have been able to find a way to save his wife from dying in her childbirth, but after waking up in the black suit, he realizes that his own fears ends up killing her.
 * Though Anakin is not necessarily blamed (again) even after his redemption and demise, the impact of both his tragic mistake and evil actions as Vader proved to be greater than either he or his son Luke expected: Ben, Luke's nephew and apprentice as well as Han Solo and Leia's son, whom upon learned Anakin's dark past as Darth Vader, decided to embraces the dark side and became Kylo Ren, causes major setback on Luke's dream to restore Jedi Order to the point that he went to exile until Rey discovered him and give him a hope.

Live-Action Films

 * Carlito Brigante: Carlito's case mostly involving poor judge on a character that he supposedly either spare or not: Carlito had the troublemaking Benny Blanco at his mercy but let him live because he was a reformed man. Carlito also knew never to trust Pachanga and said he'd take care of him but he never found the time.
 * Claire Dearing: Claire's mistake that potentially caused Jurassic World theme park nearly destroyed by Indominus' rampage was her poor judgement on the dinosaurs where she believed them as mere little more than beasts with no intelligence and gives Owen cold shoulder on her poor management on Indominus. That's until she realized that whatever treatment that she arranged for Indominus wind up destroyed what has left of her innocence where the hybrid killed many people by manipulating pterosaurs and raptors where Zach and Mitchell nearly became the the hybrid's victims as well as threatened the fate of the park.
 * Roger and Anita Radcliffe: When Roger and Anita go out for a walk to the park whilst taking Pongo and Perdita with them, they didn't take enough precaution in ensuring Pongo and Perdita's pups' safety from dalmatian kidnappers: Two bandits named Jasper and Horace Badun who are watching them from the shadows has study the whole situation in their house as they easily break in, lock Nanny in the cupboard, and stole the puppies without leaving traces. When they returned home, Roger and Anita feels saddened with what has happened and their mistakes that gave the thieves advantages. Fortunately, a group of farm animals whom knew where the thieves hiding at able to save the day by incapacitate the thieves includes their leader Cruella de Vill one by one not only to save Pongo and Perdita's puppies, but also other kidnapped homeless dalmatian puppies.

Comics

 * When Peter Parker arrogantly refused to stop for five seconds to stop a crook rushing past him, telling the cop chasing the guy it wasn't his job, his life was changed forever; the same crook would later murder his uncle. While that mistake defined what he is, the lesson was learned in the harshest way possible.

Cartoon and Animated Films

 * Marlin: Out of his own fear over Nemo's safety and trauma over his loss by an agressive barracuda that massacred his eggs and killed his wife Coral, when he tries to advices Nemo to avoid the diver's boat, his overprotectiveness and fear get his best on him and resulting his son provoked to rebels and taken away by divers. Nemo managed to prevent him from repeating the same mistake when rescue Dory and unfortunate fishes whom snared by fishermen's net.
 * Anger: In a desperate attempt to make Riley happy again, Anger comes up with an idea of running away from San Francisco and return to Minnesota. However, the idea of running away proved to be a tragic mistake as the emotion console began shutting down and Riley was turned into an apathetic person without any feeling at all. When Joy and Sadness returned, Sadness was able to fix the console by removing the idea so Riley can change her mind.
 * Megara (Disney): Motions of events that led to Megara's heroic sacrifice that caused his current lover Hercules mourns over her dead body can be traced back from when she made a deal with Hades to save her first boyfriend's life (which in TV Series revealed as Adonis). Unfortunately, he later left her for some other woman, abandoning her and leaving her heartbroken and a prisoner of Hades. This traumatized her so much that she became bitter, though when finally met Hercules whom became her true love, she finally given chance to redeemed herself by sacrificing herself to save Hercules where the process not only to allowed the demigod to reclaimed his powers and freed her from her pact with Hades, but also resulting her demise. Fortunately, she later get better after revived by the demigod whom reclaimed her soul.
 * Mr. Incredibles: Because he arrogantly detested Buddy Pine's attempt to aid him and recklessly saved a suicidal man whom doesn't wanted to be saved as well as disasters that he unintentionally caused, he not only forced all Supers to retires (which somehow also decrease the crime rate), but also turned Buddy into ruthless suervillain Syndrome whom sought to make the term "superheroes" redundant to the world which also includes genocide on all Supers, leaving Mr. Incredibles, his family, and Frozone as the presumed last survivor of Supers.
 * King Triton: Because he hated humans and learns that his daughter Ariel has fall in love to a human whom she saved from drowning that he loses his temper and begins destroying her collections and completely ignoring her pleads to stop, but he then realizes what he has done and leaves her in shame, this also lead Ariel to search of Ursula the SeaWitch.
 * Wyatt: Wyatt's tragic mistake was his show-off habit not improved the trouble that he and his friends face, but rather worsen it instead.
 * Shifu's tragic mistake prior to events in Kung Fu Panda was during trained Tai Lung, he was so proud of his talent that he forgot to taught his said student humility, which contributes to the tiger's fall from grace when it's revealed that he was not meant to be Dragon Warrior. Master Oogway also pointed out that due to the said mistake, Tai Lung's arrogance turned into the very darkness that consume his heart and made him evil.

Anime and Manga

 * Sayaka Miki: Though it could be said that Sayaka Miki's deal with Kyubey in the first place is the mistake that leads to her downfall, her actual act of Hamartia is when she refuses to cleanse her Soul Gem, because she's full of despair from seeing Kyousuke Kamijou (her childhood friend) accepting Hitomi Shizuki's love confession. Her despair not only turns her into a witch, but also leads to her and Kyouko Sakura's death.
 * Harogumo Ootsutsuki: He realized that he has done this by having Asura as his one and only successor, unknowingly allowed Black Zetsu worsened the rivalry between Asura and Indra that would lasts for generations everytime they reincarnated into their descendants. This ultimately undone with the brothers' latest incarnation, Naruto and Sasuke, eveuntually ended their feud for good.

Live Action TV

 * Supernatural:
 * Dean Winchester: Threatened Sam Winchester that he would sever their bonds and walked away from his life as means to convinced him not to drink more demon blood and abandoned Ruby so Lucifer won't free, but backfires with Sam became succumbed with her influence. When Dean decided to rekindle their relationship, he was unable to prevent Sam from freed Lucifer.
 * Castiel believed that he doesn't need Winchesters' help when tries to settle things right in Heaven since many angels prefer to start-off apocalypse, he resort to seek the cooperation from demon for the gateway of Purgatory, kills several angels whom happened to be in his way, and absorbs every single one of souls in Purgatory to put an end of the civil war. He even decided to took over God's role over the management of the universe, only to realized that Leviathans, whom he accidently absorbed, used him as means to entered Earth and killed him from the inside.

Western Animation

 * In Teen Titans 2003 TV Series, in a desperate attempt to control her earth powers, Terra chooses to join Slade whom promised her to trained her after failed to have Beast Boy keep this from other Titans, which turned out costs her friendship and love with Beast Boy.
 * Steven Universe: Steve's tragic mistake which endangered his friendship with Garnet is refuses giving the mirror to her and in blind anger, hits Garnet in the heat of their argument.

Video Games

 * Reasons why Kessler, Cole's alternate future self goes back in time to hardened his past self's life was due to his tragic mistake where he chooses to flee along with his family when The Beast emerges and killed all life in spite the fact he actually have powers to fought him, resulting Kessler the last lifeform on earth due to The Beast's rampage.

Literature

 * Seerow, the Andalite Prince whom feels pity on Yeerks, parasitic slugs whom can only see, hear, or interact with their surroundings when possessed a creature, provided them Andalite Technology in hopes of making their life better. This unfortunately led the Cataclysmic event called Seerow's Kindness, where the evil Yeerks emerges and began their crusade for enslaving all lifeform as much as they could in the entire universe.
 * Harry Potter:
 * Harry Potter refuses to be taught Occlumency by Snape because of their hatred and mistrust for each other where much to Harry's shock, their said hatred and mistrust for each other were caused by Snape's own bad history with James Potter long before James himself changed into better man due to his love with Lily. This allows Voldemort to send him a false vision of Sirius held captive so that Harry could run to his rescue. This results in the all-out fight that ends with Sirius' death, since in his impulsiveness Harry also neglected a device which would have allowed him to communicate to Sirius immediately and verify that he's fine.
 * Severus Snape also have tragic mistakes of his own, where he ruined his friendship (and any chance for romance he might have had) with Lily when he chose the Dark Arts and became a zealous Death Eater. He then relayed a prophecy to Voldemort which cause him to target Lily's son, resulting in the murder of her and James. While he eventually decided to be the ally for Harry from there on, his own personal hatred with James winds up worsen his relationship with Harry until he eventually revealed his true agenda when Voldemort killed him for mistaking him as the master of Elder Wand that led Harry to forgive him. In certain point of view, had Snape put aside his difference with Harry (which in turn convinced Harry to do the same) from the moment where they met in Harry's first year in Hogwarts, he won't have to failed in taught Harry Occulumency that winds up resulting Sirius' death nor have hostilities that they had now at least until Snape seemingly murdered Dumbledore which actually done as mercy kill.
 * Among unfortunate mistakes that Albus Dumbledore ever done, the most tragic one was tempted to start a plot against Muggles with Grindelwald, and in his lust for power caused his sister's death when his brother confronted Grindelwald.
 * Horace Slughorn's tragic mistake was allowed the young Voldemort Tom Riddle to flatter him too much and revealed to him information about making seven Horcruxes for maximum power, which resulting deaths that the future Dark Lord required to create his said Horcruxes, Hagrid being reduced to the gamekeeper as the partially succeed framing attempt as means to cover his tracks, and Lily's own death. Fortunately, he had the chance to redeem this through her son Harry whom working on helping Dumbledore' investigations.
 * Sirius Black' tragic mistake was when he let Wormtail to be the secret keeper to protect Potters, only for Wormtail betrayed them and brutally massacred innocent muggles and framed the crimes to him, resulting him sent to Azkaban until he uses his Animagus powers to escape. Other mistake would be mistreating his house-elf Kreacher, resulting the house-elf betrayed him and leaving him to die at hands of Bellatrix.

Quotes
"Fear: Guys, we can't make Riley feel anything. Anger: What have we done?"

- Anger realizing his tragic mistake which literally shuts down the console.

"Darth Sidious: Lord Vader... can you hear me? Darth Vader: Yes, Master. [Vader looks at Sidious] Where is Padme? Is she safe? Is she all right? Darth Sidious: It seems in your anger, you killed her. Darth Vader: I...? I couldn't have! She was alive... I felt it![Vader growls, and his Dark Side strength crushes everything around him in the room. He frees himself from the metal stretcher, and steps off. Palpatine has a smirk on his face] Darth Vader: Nooooooooooooooooooo!"

- Exchange between Vader and Sidious where Vader, formerly Anakin, realized that he ended up killed Padme

"Tigress: Shifu found him as a cub and he raised him as his son. And when the boy showed talent in kung fu, Shifu trained him. He believed in him. He told him he was destined for greatness. It was never enough for Tai Lung. He wanted the dragon scroll. But Oogway saw darkness in his heart and refused. Outraged, Tai Lung laid waste to the valley. He tried to take the scroll by force and Shifu had to destroy what he had created. But how could he? Shifu loved Tai Lung like he had never loved anyone before. Or since."

- Tigress explaining to Po about Shifu's tragic mistake in training Tai Lung was not to taught him humility, which resulted him in becoming an arrogant and heartless villain.

"The dead remember our indifference; the dead remember our silence. I came here tonight to be congratulated. But today, when I visited the Red Cross camps, overwhelmed by the flood of refugees fleeing the horror of Kazakhstan, I realized I don't deserve to be congratulated. None of us do. The truth is, we acted too late. Only when our own national security was threatened did we act. Radek's regime murdered over 200,000 men, women and children and we watched it on TV. We let it happen. People were being slaughtered for over a year, and we issued economic sanctions, and hid behind the rhetoric of diplomacy. How dare we? The dead remembered. Real peace is not just the absence of conflict, it's the presence of justice. Tonight I come to you with a pledge to change America's policy. Never again will I allow our political self-interest to deter us from doing what we know to be morally right. Atrocity and terror are not political weapons, and to those who would use them, your day is over. We will never negotiate. We will no longer tolerate and we will no longer be afraid. It's your turn to be afraid."

- President James Marshall revealing a tragic mistake that cost countless lives.