Saw this on Cosmic Variance. We are all still here thanks to the common sense, nerve and intelligence of the Soviet military officer Stanislav Evgrafovich Petrov. See the details on Cosmic Variance, but basically he decided that a computer alert that the USA had launched a few missiles, likely with nuclear warheads, was a computer error rather than a real launch. So he failed to pass the info up the chain of command and therefore most likely averted a retaliation and possibly a full nuclear exchange.