On October 30, 1938, as a special Halloween event, Orson Welles and his Mercury Theatre broadcast a radio play of H.G. Wells’ The War of the Worlds. The 60-minute presentation of fictional news bulletins suggested that Martians had landed in Grover’s Mill, New Jersey, and were attacking New York City.
The acting and presentations were so convincing, many listeners believed aliens were actually invading Earth, and panic ensued. At least, that's the story.
The truth is that the Mercury Theatre broadcast actually had very few listeners; it was not the most popular radio show in its timeslot. Few of its listeners failed to notice the repeated notices of the broadcast being fictional. Most of the remaining, confused listeners thought initially thought the "Martian invasion" was a natural disaster or news of a Nazi attack—neither an unreasonable fear.
H.G. Wells met Orson Welles a year later, and shared a prank with the director. "Are you sure there was such a panic in America—or wasn't it your Halloween fun?"