It has probably happened in the middle of your favorite show. Right at the most exciting part, the television flashes to a rainbow bar graph and all sound halts. Three ear-splattering screeches grab your attention and hold it with a full 15 seconds of blaring tone. If you're lucky, you'll hear, "This is only a test." If not, the Emergency Alert System will notify you of a flood, an earthquake or a nuclear accident. In 1997, the EAS replaced the Emergency Broadcast System, which for more than 30 years interrupted shows from Flipper to Dallas to Cheers. In February 1971, a civil defense operator accidentally played the wrong alert tape, causing momentary system chaos. One Massachusetts man thought, President Nixon has pushed the wrong button. He can be forgiven for the assumption: the alert referenced "EAN message one," the designation that would be used in the event of a nuclear attack. Regular television resumed without serious panic.