To launch a Minuteman in those days, one had to "unlock" the missile by dialing in a code -- the equivalent of a safety catch on a handgun. However, Blair reports, the U.S. Strategic Air Command was worried that a bunch of sissy safety features might slow things down. It ordered all locks set to 00000000 -- and in launch checklists, reminded all launch officers like Blair to keep the codes there. "So the 'secret unlock code' during the height of the nuclear crises of the Cold War," Blair says, "remained constant at 00000000."
http://www.themoscowtimes.com/stories/2004/05/27/007.html

Weeks later, on September 26, 1983, at a half-hour past midnight, Petrov was watching horrified as a warning system he had helped create reported five U.S. missiles launched and headed toward Soviet territory.

Blair says this was the closest we've ever come to accidental nuclear war. "By all rights we should have blown ourselves to bits by now, but good luck and good judgment up and down the chain of command have spared us this fate ... so far."

All the data checked out; there was no sign of any glitch or error. Yet Petrov says, "I just couldn't believe that just like that, all of a sudden, someone would hurl five missiles at us." And: "I imagined if I'd assume the responsibility for unleashing the Third World War -- and I said, 'No, I wouldn't.'"

Petrov declared it to be a false alarm -- not because he had any evidence of that, but because he wanted it to be false.