I don't have sources because it was a long time ago and too much trouble to look up, but one incident occurred in the 1980s in Europe. It was when Nato was staging a big military exercise. The Soviets knew that but something still triggered their automated defenses. It was only the action of one officer who knew something was wrong that prevented a retaliatory strike that probably would have started WWIII.
The other one was during the Cuban missile crisis when a Soviet sub lost contact with home and nearly nuked the American fleet because it thought the war had started. Again, it was just one Soviet officer who felt something was wrong and prevented the launch.
That one was mentioned in whatever article I read. I must look for it tonight, it was fascinating and scary