In the Magical Girl Lyrical Nanoha fanfic Infinity, 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 an I Surrender, Suckers, but Thor anticipates it and redirects Loki's own Soul Seal spell back on him, killing him.
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.
In Adventure Time fanfic, Alex, the titular antagonist provokes her opponent, Marceline, to spit at her coat, the same way she spitted at Princess Bubblegum's jacket at "What Was Missing". When she does it, the Witch uses a magic spell to put it at her gray top instead. Much after that, she has Finn and some wolves attacking her, the same way she did it to the former in "Go With Me".
Happens to a nameless griffon Mook in Ace Combat The Equestrian War. When he tries to bomb Stalliongrad, the pegasi manage to send the bomb he drops back at him, coupled with Derpy knocking the soldier toward the incoming bomb that went off.
The sequel mirrors this, with an Exile Pegasus trying to bomb Ponyville. This time, it's Sunburst getting the kill: he delivers a powerful kick to stun the bomber, and then catches the bomb and throws it at his foe (who's already falling out of the sky) to finish him off. There's even a brief cut to the bomber's perspective, as he helplessly watches his own bomb coming at him! And to top it all off, there's Sunburst's Pre-Mortem One-Liner:
Sunburst: "You forgot something!" *KABOOM!*
In Nerve Damage, after Shirou breaks the Heaven's Feel, Zouken forces Sakura to summon another servant besides Rider, even over her desperate desire not to fight Shirou. Cue her summoning an Assassin with just the power set to kill him off for real while leaving Sakura alive, who disdains the Grail and all who seek it as blasphemers, and who sees Zouken as just the type of monster she was designed to fight.
Time Lords and Terror: Hydia, who spent the whole story trying to release the S'Muz, ends up being its first victim. Also, the SíMuz itself is defeated when the ponies use their Elements of Harmony to absorb both the PKE it stole and the PKE that makes up its body and disperse it into Equestria.
Queen Of All Oni: Jade has Verde construct her an enchanted dog carrier to contain the Oni-masked Scruffy while speaking with the Bat-General; Tohru later uses this dog carrier to capture her (her minions save her, however). On top of that, Tohru reverse-engineers the magic involved to create sutra scrolls capable to restraining Oni.
Later, when Lung, Daolon Wong's former apprentice, captures Jade and tries to bind her and thus her Shadowkhan to his will (which also counts as a Critical Research Failure, since Ikazuki's curse only allows her to summon Left and Right), his defenses are no match for her servants and he is Killed Off for Real for torturing Jade and nearly killing her. No tears were shed.
During the fiance arc of A Growing Affection, Hinata's grandfather Hyouta uses the Hyuga Clan's laws and traditions to try to break up Naruto and Hinata. When he thinks he has won, he says the wrong thing, and those same laws get turned on him. The result is Hyouta getting curb-stomped by Naruto, and then losing his power and status.
Several villains in Children of Time. Notably Dalek Caan and Colonel Marshall in "The Manhattan Conspiracy"... Caan wants to study a Time Lord's regenerative use of the Time Vortex? Give 'im the Time Vortex, then... And Marshall is quite taken with the power of the atomic bomb — the two Time Lords of the story plan accordingly.
She also gets a double dose because her plan of merging with every version of Nightmare Purgatory the moment of their birth results in all the alternate Nightmare Eclipses being linked to her, so her defeat ends up taking all of them with her.
Earlier in Dark World, Rancor showed awareness of this trope and avoided it as best she could, refusing to use her Concept Killing Spear in battle with the Elements of Harmony, and later destroys it after using it to steal Destruction's power from Discord, so no one can ever use it against her. That said, her insistence on fighting Dark World!Spike backfires when he figures out how to get around her immunity to violence to actually hurt her.
In Seven Little Killers, Canada has been dumping the bodies of the people America allows to be killed in the river. Unfortunately for him, he turns out to be one of the people America decides to let die. Guess where he winds up drowning?
The Calvinverse villain Rupert Chill usually disguises himself as a human criminal of the same name whenever he'd arrive on Earth. ("The Five Calvins" implies this was intentional on alien!Rupert's part.) This comes back to bite him at the end of Calvin & Hobbes: The Series' Season 5 premiere when Calvin arranges for Rupert to be arrested by police while wearing his human disguise, and they dismiss his frantic attempts to tell them who he really is as insanity.
In the The Rise Of Darth Vulcan, Big Boss, a dragon who obtained a massive hoard of gems by enslaving and abusing the Diamond Dogs, is defeated when the title character uses his magic to form talons from the gems and metals of that very hoard , grabs him, and smashes him around repeatedly with them. And then, Vulcan seals his mouth and nostrils shut with that treasure, causing the backfire that blows Big Boss' head off.
The barrier in The Conversion Bureau: The Other Side of the Spectrum does this to the Solar Empire ponies. While it's their main trump card (as they'd have been destroyed within days without it), it's made abundantly clear that the Barrier is, for lack of a better term, screwing them over. Since it destroys everything made by humans, like agriculture, infrastructure, buildings and the information on how to make those things, the TCB!ponies that are trying to colonize earth are forced to build everything from scratch (for example, they can't use human steel to make railroads since that gets destroyed) while also diverting their resources to the war. And thanks to the massive influx of newfoals, production of food (and virtually everything else) is at an all time low.
The Fujin missile which took down the Great Equestrian airship was able to do so by absorbing all the magic the TCB!ponies used in a failed attempt to stop it.
TCB!Cloudchaser was hit by a poison that works like this — it absorbs any magic in the area and kills the person or pony slowly. Thankfully, Discord saves the day.
In Harry Potter and the Wand of Uru, Flint's decision to have Wood injured badly enough to be removed from the game comes back to bite him when the replacement Keeper (Hermione) proves to be just as good and have better gear. Then she and Harry knock the opposing Seeker out of the game, making it impossible for Slytherin to not lose horribly since the only way to forfeit is for both captains to agree and Wood's still in the hospital wing.
Alicornundrum: Duke Blueblood is ultimately brought down when Nyx and her friends break into his home and find proof of his conspiracy against the throne, which is enough to horrify his son Prince Blueblood enough to help expose him. And all this only happened because the Duke spread photoshopped pictures of Nyx and her friends in lewd conditions as part of a petty smear campaign.
Harry Crow: Lucius Malfoy has 24 hours to withdraw the contents of his Gringotts vaults and anything left behind after then will be unreachable until his son becomes of age. Lucius is forced to miss the deadline because the Ministry has him under arrest and his wife can't do it for him because he never gave her permission to withdraw more than one hundred thousand Galleons per day.
Subverted in Pyrrha(c) Victory. One of the characters buys the equivalent of a love potion, intending to use it on the boy she likes...and eventually decides not to, reasoning that it would be wrong, and that it probably wouldn't work anyway. Despite this, she trips and accidentally doses herself with it. Moral of the story? Illegal substances are nobody's friend.