The Mal-Observation Report by Richard Hodgson and SJ Davey

In the late 1880s, a young self-taught amateur magician named Samuel J. Davey gave a series of private séances in England for small groups of invited sitters in which he duplicated mediumistic feats, particularly spirit-slate writing (or psychography) under apparently test conditions. He was not paid for his demonstrations, asking only that the sitters write down their recollections of what they had experienced. They were neither told that he was a magician, nor that he had psychic powers, merely that they should watch out for trickery and draw their own conclusions. To his surprise, their recollections, even shortly after the fact, differed markedly from what had actually transpired, leading them in most cases to leave convinced of his psychic abilities, even when he later told them otherwise.