Most Americans are familiar with the Emergency Alert System (EAS), which replaced the Emergency Broadcast System (EBS) in 1997. In the age of 24-hour cable and internet, you will rarely hear the weekly tests anymore unless you are up at 2AM on a Sunday, but you will probably recognize the alert tone. That's the tone that is illegal to broadcast unless you are authorized. I used to do it all the time.
Let me indulge in a personal story about the Emergency Broadcast System. I once worked at a large radio station that was an EBS relay station, meaning we were authorized to send an alert and take over the signal of all the other stations around us in case of a nuclear attack. This was considered a real possibility during the Cold War. I received the necessary training on how to use the system, and would relay weekly tests ("This is a test. This is only a test"). Then sometime, I like to think 1991, that state decided since the Cold War was over, we would use the EBS to alert people about local emergencies. I received more training.
Meanwhile, my husband at the time worked at a competing radio station, so we never discussed radio business. He was promoted to program director. His very first day on the new job was the day I received my first real EBS emergency, just a few weeks after the criteria was changed. The alert went out with my voice saying "This is not a test." It was a severe storm warning, which I thought was silly because it was obvious by looking out the window and what can you do about it? I learned later that my husband, who had not yet been told about the change in criteria because his station was not a relay station and he wasn't a part of management until that day, almost had a heart attack because the Soviets were launching a nuclear attack on his first day in charge.
-via Boing Boing