“On September 26, 1983, Lieutenant Colonel Stanislav Petrov was on duty at the bunker outside Moscow that monitored the Soviet Union’s Oko early-warning satellite system, when the alarm bells went off shortly after midnight. A satellite was indicating that the United States had fired five ballistic missiles at the Soviet Union. His orders were to pass the warning up the chain of command, which would approve the launch of a nuclear counterstrike. But he didn’t. A satellite was later discovered to be the culprit, mistaking the sun’s reflection off the tops of clouds for a missile launch. The computer program that was supposed to filter out such information failed to do so.”
http://www.rferl.org/content/soviet-colonel-averted-nuclear-war/25118749.html