Who: Meyer Prinstein
Olympics: Paris 1900

Leading the long jump competition heading into the final round, American Meyer Prinstein, who was Jewish, would not compete in the final because it was held on a Sunday. Prinstein claims that he had an agreement with his main competitor, fellow American and Christian Al Kraenzlein, that neither would compete. Apparently it wasn't a very good agreement because Kraenzlein participated and beat Prinstein by a quarter of an inch. Prinstein was decidedly pissed off and punched Kraenzlein in the face (or was prevented from doing so, depending on whose account you believe).