The Swiss Spaghetti Harvest
One of the greatest April Fools pranks of all time was pulled in 1957 by the BBC, of all institutions. Aired as an ordinary episode of the renowned series Panorama, it purported to be a documentary about "a family from Ticino in Switzerland carrying out their annual spaghetti harvest. It showed women carefully plucking strands of spaghetti from a tree and laying them in the sun to dry."
It sounds ridiculous now, sure, but back then many people had either never heard of spaghetti or had only ever had it from cans, and the episode was shot in a completely straightforward fashion and narrated by the respected journalist Richard Dimbleby. Hundreds of people called the BBC to ask where they could purchase spaghetti bushes for themselves!