In Infinity, a fanfic derived from Magical Girl Lyrical Nanoha, Lindy spends most of chapter 25 getting smacked around by Precia. Near the end, she reveals that she's been sealing Precia's magic the whole time, and electrocutes her with all the magic Precia used during the fight.
Around the climax of My Little Avengers, this happens twice. First, when it appears as though the villains have won, Trixie attempts to betray Loki, only for him to turn her own magic back on her. She survives and escapes, but is left horribly scarred. Later, during Loki and Thor's Final Battle, Loki pulls a I Surrender, Suckers, but Thor anticipates it and redirects Loki's own Soul Seal spell back on him, killing him.
One Star Trek: The Next Generation Fan Fic involving Q creating a second Jean Luc seems to be written just to conclude with Data telling him "I'd say you were hoist by your own Picard."
In Winter War, the Barragan Fragment's power to age things to dust extends to its blood. The fight is going badly when Isane guesses that it might be vulnerable to its own power. At her suggestion, Ukitake and Soi Fong put its blood on their swords before attacking again. Luckily for them, it works.
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