In one of my all-time favourite hoaxes, in 1957 the BBC managed to convince the British public that spaghetti grows on trees.
The segment, narrated by veteran broadcaster Richard Dimbleby, showed a Swiss family harvesting spaghetti from trees, drying it in the sun, and then serving it up for dinner. All this was presented completely straight-faced, and viewers fell for it. Hundreds called in to the BBC to ask about acquiring their own spaghetti tree.