Bell's theorem, proving Einstein's intuition was wrong, and that quantum mechanics does have some sort of a spooky action at a distance, (this or that the moon is not necessarily there when you don't look at it). E.g. we have either no locality (cause and effect can't propagate faster than light), or no realism (things don't have a "realness" until measured, e.g. the wave function mode of particles), or superdeterminism (everything is predetermined, no free will, nothing is random, not even the random behavior of quantum particles that seem the most random thing in the world)