For me, flat Earth. The sheer skill necessary to bribe or convince so many governments to gaslight their population, the incredible optics and geodesics work necessary to fake a curved horizon in a plane window and have consistent flight durations, and the maintenance of shadow power across millenia, paints the picture of a group of people way beyond what human competence can achieve.

Of course, there are simpler examples, like John Titor, or Pierre de Fermat’s last theorem proof that he would have done all in his head.