It was a mistake, but a very scary one. It happened in Feb. 1970 and the station, WOWO in Ft. Wayne, Indiana and its listeners had every reason to believe it was real.

The aircheck from that fateful day certainly makes it sound like Armageddon was close at hand. At one point, the announcer, Bob Sievers, asks any news staff to report to the station immediately. 

A fascinating bit of radio history I'd never heard before. If you ever wondered what this might sound like, this will give you a pretty good idea. Scary stuff.

The Day an Emergency Broadcast System Error Spread Nuclear Fear