A "failure of Friendliness" scenario in which an AI asked to solve the Riemann Hypothesis turns all the matter in the solar system into computronium, exterminating humanity along the way.
This idea possibly originates---with a slightly different phrasing---from Marvin Minsky.
In variant forms, the Riemann Hypothesis Catastrophe is a subclass of the subgoal stomp error, the Devil's Contract problem (both diabolic and golemic), and the "emergent pressures" scenario a la "The Two Faces of Tomorrow."
This scenario is dscussed in more detail in Creating Friendly AI.