Los Alamos National Laboratory · Public domain

Louis Slotin, a Canadian physicist on the Manhattan Project, performed hands-on experiments bringing plutonium cores to the edge of criticality, using a screwdriver to hold two hemispheres of fissile material a hair's breadth apart in what colleagues called "tickling the dragon's tail." On May 21, 1946, the screwdriver slipped, triggering a supercritical reaction whose radiation killed him nine days later. He was credited with reacting fast enough to spare colleagues in the room a similar dose, though physicists have since said the accident followed directly from his own disregard for established safety protocol.