On Halloween 1992, the BBC aired Ghostwatch, a 90-minute special presented as a live, scientific investigation of a haunted house. It was, of course, strictly a faux doc—but it still managed to confuse more than a few viewers who believed they were watching real terrors caught on tape.
Source: io9 – US Viewers Can Finally See the Legendary, Pants-Wettingly Scary British Mockumentary Ghostwatch