In February 2013, a Montana TV station was interrupted with a real-seeming emergency broadcast system update that bodies were rising from their graves, and they were considered extremely dangerous. A few people called the police to verify if the news was true. The TV station itself said that the message didn't originate from them, and officials never really figured out who the mysterious hoaxers were, just that they orginated from some "overseas source." Here's the admittedly convincing emergency alert.
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