Nice Job Breaking It, Hero/Fan Works

In Light and Dark The Adventures of Dark Yagami, the girl from the bus kills Dark and gets hailed as a hero for doing so, but he goes to the Shinigami world after dying, kills Blud and becomes king of the Shinigami. He even thanks her for killing him and enabling his ascent.
In Modurn Warfare 3 The Dead Rise, ~~Soap~~ Soup throwing a knife into General Shepherd's eye triggers his demonic powers, enabling him to come back even more powerful than before.
Nia my of Fuck the Jesus Beam decides to deal with the Bombignat threat by releasing O.B.A.M.A. from his prison, despite him having started 95 World Wars as president and, as it is later revealed, being the source of the Bombignats. Upon his release, O.B.A.M.A. says that he will reclaim his domain, and takes control of the Bombignats.
Okay, Luminosity's only difference is that Bella's a different character, so naturally the differences for good or ill will be her fault...village murdered because of her? And now the Volturi have shapeshifters on their side? Because Bella thought it would help them. Oops. And Elspeth is following in her mother's making-the-Volturi-stronger footsteps.
Tends to be a habit with Naiki Satomi from Divine Blood. Some specifics:
- Underestimates the Karate Club guys and gets in a position where she has to use a chi burst to defeat them in public, while Gauron is spying on Kaname nearby.
- Naiki's fear of flying combined with a terrorist takeover of the plane she's on causes her to go on a fear-crazed berserk as soon as the plane lands, which leaves her captured and used as a hostage to insure Deimosu's surrender.
- Goes back to help Kurz's fight against the Venom Chodarl, splitting the escape party and causing Eija, Kaname, Deimosu and Sousuke to hesitate long enough figuring out where she went for the Musk to attack and force Eija, Kaname and Sousuke to miss the escape.
- Tries and fails to rescue and protect Tessa from the Behemoth terrorists.
- Uses a black magic spell that causes her to touch Tessa's soul, starting the mind merge process and also indebting her to a Demon that makes Mara queasy. Twice.
- In backstory, Naiki's intense fear of flying is due to an...interesting 10 meter trip through a hangar that occurred when she tried to operate a helicopter as a young girl.
- In the fight against the Behemoth, uses her summoned tentacles to throw Mao into a pile of debris, almost effectively taking her out of the fight.
- In the Martial Arts Takeout Race, loses control of her newly made Torrent of Debris spell.
- Before getting one of Persephone's peaches, she spends a lot of the fight against Poseidon being rescued by the older, human martial artists.
While not much of a hero, Vash of Christian Humber Reloaded kills a dragon named Le-Hung Doe as part of atoning for killing another dragon, but learns that by killing him, he has started a Ying-Yang War, in which he must fight demons without weapons.
There are not words to describe Celestia's epic screw-up in Past Sins. Suffice it to say that if she had just held back, almost none of the really bad things would have happened.
- Because Twilight didn't stop Celestia, Nyx becomes convinced she was in on it.
In Connecting the Dots, when Kakashi teleports himself to The DCAU to try and rescue Naruto and the Konoha 11, he appears right in front of Lex Luthor. This is particularly bad as the Sharingan was the last thing Luthor needed to complete the Anti-Life Equation.
Chosen To Destroy, written as a Take That at Buffy the Vampire Slayer and Angel, which has the First Evil show up after the Season 7 Buffy finale, basically to gloat over how it's going to open all the world's Hellmouths and kill all the Slayers in the world, and to rub it in that it's all Buffy's fault - while some of it was the First's manipulations, it makes it very clear that the main reason it's going to win now is because of Buffy's ego and stupidity (and Angel's to a lesser extent).
Inner Demons: If Celestia had told Twilight about the prophecy sooner, or broken the news to her differently, it's possible that perhaps Twilight's transformation into the Queen of Darkness could have been slowed or altogether prevented. At the very least, Twilight wouldn't have freaked out and turned Celestia to stone in a fit of anger at the revelation.
- Twilight actually pulls this on herself. After the above incident, she convinces her friends to lock her up in the library basement while Rarity searches for a way to stop her descent into darkness and free Celestia. Unfortunately, since Spike is busy helping Rarity and Twilight's all alone, there's no one to wake her up from her nightmares, wherein the Queen persona consumes her completely.
- A little later on, Sweetie Belle is on the verge of a Heel Realization, when Rarity's unfortunate wording during a conversation hits an emotional trigger Queen!Twilight had previously planted, causing Sweetie Belle to fully embrace her Face Heel Turn and reject Rarity as her sister.
Everything in Imperfect Metamorphosis can ultimately because of Team 9, and Cirno specifically. They were responsible for both releasing the Shadow Youkai and getting Yuuka involved, and their constant meddling continued to make things even worse. Other characters caused a lot of headaches too (especially Eirin and Yukari), but nowhere near to that extent.
Eri's Game: Because of her fight with Higashizawa, Shiki unknowingly let her best friend, Eri, got her death wish. Oops.