Don Sensing links to a story about the man who said "No."
Stanislov Petrov was in charge of the Soviet Union's DEW system on September 26, 1983. Soviet pilots had just shot down KAL 007 three and a half weeks earlier.
Just after midnight the Oko ("eye") satellite array indicated a launch of five Minuteman II missles from Montana. Petrov had just a few minutes to warn the Soviet leaders who had to decide on a response, but something felt wrong to Lieutenant Colonel. Why only five?
So he told those leaders it was a false alarm: "I imagined if Id assume the responsibility for unleashing the third World War and I said, no, I wouldnt.