Tragic Mistake

In a formal Tragedy, there is a specific scene where the Tragic Hero is given a clear choice, and they choose poorly. Often this wrong choice can be blamed on the hero's Fatal Flaw, but sometimes they just get screwed over by fate.

This moment may not be obvious at the time, but looking back, it becomes clear that this moment was crucial to the hero's tragic downfall.

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.

As the Tragic Mistake is one of the most crucial moments in the 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.
 * 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.

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, when he tries to advices Nemo to avoid the diver's boat, his overprotectiveness and fear ended up provoke his son 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.

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.

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, only to realized that Leviathans, whom he accidently absorbed, used him as means to entered Earth and killed him from the inside.

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.