Do You Believe?

Wiki:

Project Alpha was an elaborate hoax orchestrated by the stage magician and skeptic James Randi. It involved planting two fake psychics, Steve Shaw (known as Banachek) and Michael Edwards, into a paranormal research project. During the initial stages of the investigation, the researchers came to believe that the pair's psychic powers were real. However, more formal experiments, as well as criticism from both the parapsychology community and Mr. Randi himself, led them to dismiss their initial trust. The hoax was later revealed publicly.

The success of Project Alpha led Randi to use variations of the technique on several other occasions. Perhaps the most famous example led to the downfall of TV evangelist and faith healer Peter Popoff, when Randi had a man pose as a woman with uterine cancer, which Popoff happily "cured." In another example, Randi hired a performance artist to pose as a channeller known as "Carlos," who was presented on Australian TV and soon had a wide following. After this hoax was exposed, the artist was constantly approached by people who believed him to be genuine, even if he told them directly that he was an actor.