In 1983, Stanislav Petrov was on duty at a Soviet nuclear early warning centre, when computers warned of five incoming missiles. Rather than escalating it up the chain of command as protocol required, Petrov surmised that the Americans were unlikely to attack with only five missiles. Courageously, he reported a system malfunction instead. It turned out that computers had incorrectly interpreted sunlight reflecting off clouds as missiles. Petrov's instincts and healthy scepticism of the infallibility of technology may have averted a nuclear disaster.