Meltdown and Spectre
It's not everyday that a security issue is borne out of a side-effect of underlying theory; in this case, how out-of-order execution works. It's almost as bad as if someone proved P = NP, rendering all cryptography useless on the spot.The game is changed forever, and CPU designers across the industry will likely have to can some or all of OoOE, setting performance back considerably. It can take years to reachieve performance we're used to today at the security level required.