The Cottingley FairiesIn 1917, two schoolgirls named Elsie Wright and Frances Griffiths (16 and 10 years old, respectively), produced what appeared to be substantial proof of the existence of fairies: a series of photographs showing the girls frolicing with fantastic humanoid, winged creatures in the forests of Cottingley Glen. While the jaundiced eye of the 21st century might consider them patently obvious in their deception, to the folks of the time, they became rather hotly contested and lauded.
A particular high point in the story is the appearance of Sir Arthur Conan Doyle, years after the initial publicity, where he both believed the girls and endeavored to work with them to help prove them out (and generate further publicity).
Alas, six decades later the girls (now quite old) confessed to having made the fairies up out of cardboard cutouts, and to have created the entire mess up. Well, unless they were just trying to hide the fairies from the world....