Stanislav Yevgrafovich Petrov, a Lieutenant Colonel of the USSR who decided to trust his own judgment over standing orders and did not unleash a nuclear holocaust when, in 1983, it appeared as if the US had launched a missile attack.
It's nice to know that (a) there was room human judgment in the system, and (b) the human involved made the right choice. Are we lucky or what? Things like this make me wonder about the anthropic principle. Perhaps in most universes we were incinerated and its just the happy few that had an intelligent and brave officer in charge at the right time that there are people around to celebrate.