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Mishra in Magic The Gathering relied heavily on his machines and weapons, to the point of falling to the manipulations of a cult of machine worshipers.
Then there are redirection spells, meaning that the spell your opponent hurled at your face is suddenly pointed at their own nether regions. Ow.
Truly sadistic GMs in just about any game have been known, instead of railroading difficult PCs, to give them just enough leeway to get in over their heads due to their own actions and decisions - even if those decisions had no other flaw than deviating from the GM's plotline.
How many D&D mages or characters in other games have fallen victim to their own fireballs, grenades or other highly-lethal AOE attacks?
One early 1st Edition AD&D adventure from Role Aids concluded when the hero shows the Big Bad a branch from a tree in which the villain had gotten trapped as a young boy. This triggers a panic attack in which the Big Bad cries aloud: "That tree tried to kill me! Kill me!" Unfortunately for the evil wizard, the obedient flesh golem Mook at his side takes its orders very literally, and compliantly snaps its maker's neck.
In the Greyhawk D&D setting, the tyrannical mage Tuerny was eventually tricked into getting trapped inside an artifact of his own invention: the Iron Flask that bears his name.
In 40K the perils of the warp rule does this for psykers - especially if you're a Librarian casting vortex of doom.
Large blast weapons with short ranges can fly right back at the thrower or hit nearby troops. It is entirely possible to completely miss whoever you were aiming at, and obliterate your own tank.
Several of the Skaven weapons can invoke this when they inevitably go awry. Orks still have some element of this, but is largely toned down in recent editions.
High Lord Goge Vandire, created the Sisters of Battle as his personal enforcers, but they soon turn against him and killed him when his madness went to far.
One Paranoia mission calls for a volunteer to test an experimental "traitorkiller". When activated, it explodes. This is intentional; the idea is that traitors will volunteer to prevent it from being used against them.
The Jihad era of BattleTech saw the Clans undergo something known as the Wars of Reaving, due to the massive abuse of Trials of Reaving as political weapons, effectively trimming down the holders of blood heritages among the Clans. A Reaving could be called for almost any reason, and was most blatantly used by Steel Viper Khan Brett Andrews to kill his political opponents in the open. After many long and bloody battles, a Reaving ended up being called on Andrews and his entire Clan, supported by the other remaining Clans, who destroyed Andrews and the Steel Vipers outright.
A common theme in Deadlands is the destruction of evil by its own devices. For example, the only way to permanently kill a Hangin' Judge (the damned soul of a judge who wrongfully sentenced multiple people to death for his own gain) is to hang it.
In Shadowrun, Richard Villiers set up the Trans-Latvian Enterprises investment bank to act as a shell corporation to protect his assets and he "borrowed" against his stock in Novatech to finance the corporation. Villiers had to cut ties with TLE so his assets couldn't be seized in the event something went wrong during his split from Fuchi. Free from Villiers's off-the-books direction, TLE called in his debts; Villiers defaulted on the loan and TLE confiscated 24% of his stock in Novatech.
The Dark World and Lightsworn cards in Yu-Gi-Oh! were specifically designed to turn hand- and deck-control decks against themselves, as Dark World monsters trigger effects when they are discarded, and Lightsworn monsters are powered up by the presence of Lightsworns in the Graveyard. To this end, both deck types include lots of cards whose effects include discarding - which means they themselves fall victim to this trope against a deck built around removing cards from play.