My players have a sore spot for mind flayers on account of a bit of mild enslavement that caused them to live as thralls for ten years of their young adulthood. Tonight, many, many sessions later, a benevolent trickster god offered one party member a wish. The party member immediately seized upon this opportunity to "deal with the mind flayers once and for all." This player, being a good roleplayer, spent some reasonable time hemming and hawing about the morality of genocide and the danger of upsetting the cosmic balance, before settling on "I wish for all mind flayers to lose their psionic powers." This was an excellently devious wish that enacts justice, eliminates the threat, and doesn't rise to the level of outright genocide, so the trickster god granted it.

It wasn't until later that I remembered a previous decision I'd made, unbeknownst to the players. One player is a warlock whose patron is Yog-Sothoth from the Lovecraftian universe. My version of Yog-Sothoth is the deity of creepy crustaceans like crab, Chuul, the like. His rival Cthulhu is the god of gastropods like octopus, squid, and of course mind flayers.

So when my player wished away mind flayer psionics they surely either killed Cthulhu outright or weakened him enough that the other elder gods are currently turning what's left of him into future Githyanki real-estate.

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TL;DR One of my players accidentally killed Cthulhu.