In July of 1917, two girls by the names of Elsie and Polly borrowed a camera and started taking pictures. By 1920, Doyle was asking them to take more of these amazing photos. The picture above is their first one. In total, there were five. At the time, spiritualists were accepting these as hard evidence that fairies are real. Doyle even wrote a book by the title "The Coming of the Fairies." Photographic experts came forward making various cases as to why the photographs could not have been faked. It was also argued that the two girls had no motivation for faking the photos. All of these things together lead the fairy photos to become very popular, but no more real.
This case is a great example of the many pieces of "evidence" that are often put forward for claims of the supernatural. First is the "Argument from Authority." Just because someone famous like Sir Conan Doyle agrees with a claim doesn't mean that it's right. Even the photographic experts were fooled. The simple reality of life is that no one is above being fooled. Just because a someone in a white lab coat says X is true, doesn't make it so.
They also accept the photographs and anecdotal reports as hard evidence. Photos can be faked in any number of ways. This is even more true now in the age of Photoshop than it was in the 1920's. Anecdotes are a good place to start a investigation, but are just too unreliable to base such extraordinary claims on.
In the end, the Cuttingly fairies were revealed to just be paper cutouts. It is so simple, and still it was dismissed as a possibility at the time. Before we accept fairies, maybe we should ask how even the researchers could have been fooled. Never underestimate how easy it is to trick even the experts.