In 1999 DJs at Oregon radio station KSJJ announced that the Ochoco dam had burst, threatening downstream areas with massive flooding.
What made the warning believable was that hundreds of houses in these areas had been damaged the previous year when the Ochoco Creek had flooded, so terrified homeowners who heard the news quickly prepared to flee. Later the DJs informed their listeners that it was all a joke. They had just been ‘having a little fun’. The homeowners were not amused.
In the same genre of non-funny disaster warnings, there’s also WNOR’s 1992 April 1st report in which it warned that a large build-up of methane gas was about to cause a fiery explosion at Mount Trashmore, a landfill near Virginia Beach. Residents were warned to evacuate the area, causing the local 911 to be flooded with calls. The DJs responsible for the prank were suspended without pay for two weeks.