Thanos' Snap Had a 50% Chance of Turning Him into a Gorilla (Really)
When he originally set out to acquire the fabled Infinity Stones, the Mad Titan Thanos intended to become a living god, able to toy with the fabric of time and space. Besotted with the metaphysical embodiment of Death, Thanos planned to conquer creation and sit beside his queen for all eternity, and in Infinity Gauntlet #1, he began that process by extinguishing half of all life in the universe. It was a terrible act, and one thankfully reversed by the efforts of Earth's heroes, but it also had a 50/50 chance of turning Thanos into a gorilla.
Thanos' genocide comes with this bizarre caveat because of the sheer breadth of the Marvel Universe, and the countless weird and wonderful creatures that reside within it. It's a staple of Marvel's biggest stories that ambitious actions have unexpected consequences. The Skrull Invasion turned Green Goblin into a national hero, restoring Captain America's youth transformed him into a servant of Hydra, and ending the Age of Ultron set Galactus loose on alternate realities. So despite his tampering with reality itself, Thanos' potential transformation hinged not on his ability to control his newfound power, but on the chances of killing Ken Hale, aka Gorilla-Man.
Ken Hale is a former mercenary afflicted with the Curse of the Gorilla-Man. This curse renders him immune to aging or any form of natural death, but transforms him into a gorilla. A longtime member of the Agents of Atlas and later the Avengers' traitorous head of security, Ken has used his powers to do good, even as he's grown to hate living his life as an ape. But Ken would also be the first to admit he deserves his punishment, as the Curse of the Gorilla-Man is magically passed on to whoever murders the previous bearer, whether human, Atlantean, or alien. This means that when he killed off exactly half of all life, Thanos had an even chance of killing Ken and taking the curse on himself.
As many have observed, Thanos' snap would have cost more than the number of lives he intended, given the unpredictable and chaotic consequences as half of all surgeons, nuclear technicians, and pilots vanished in the same moment, making him doubly lucky to avoid the shape-changing punishment. It's likely that Thanos would have been able to transform himself back to Titanian form had the curse actually taken hold, but the Infinity Gauntlet is guided by the users' will, so he would likely have spent a few moments in simian form before realizing the consequences of his actions.
In the wide world of Marvel Comics, there's always some clashing or forgotten elements of continuity that fans can point to - from Tony Stark apparently forgetting he knows how to telepathy-proof his Iron Man armor to Hulk's tongue changing color depending on the artist - but Thanos' near miss with gorilla-hood is one of the most enjoyable. Sadly for Ken, he didn't get to go out with the knowledge that he'd finally passed his curse on to someone truly deserving, and comic and MCU fans alike missed out on the chance to see Thanos deal with the very unexpected cost of his actions.