An officer who didn't believe the computer in 1983.
On 26 September 1983, the Soviet early-warning system reported the launch of five US missiles. Duty officer Lieutenant Colonel Stanislav Petrov had minutes to escalate the alarm. He judged it a false alarm: a real attack would involve hundreds of missiles. He was right; sunlight reflecting off clouds had fooled the satellites.
Useful: Check whether an alarm fits the logic of the threat before you act on it.
Source: en.wikipedia.org