Black Saturday

On the worst day of the Cuban missile crisis a Soviet submarine officer refused to agree to launch a nuclear torpedo, and the world found out about it thirty-nine years later.

The same day a U-2 was shot down over Cuba and another strayed into Soviet airspace over Siberia. Vasily Arkhipov's veto only became public at a conference in 2002, which is a long time for a species to go without knowing how close it had come.