The disappearing blond gene
A story hit the airwaves in 2002 when BBC News reported that German scientists had discovered that blond hair would become extinct within the next 200 years because it is a recessive trait.
Later the same year, the New York Times, in attempting to corroborate the report, discovered that no such study had ever been performed. Despite the revelation, the study continued to be cited in publications for years, including as recently as 2006. (You have to wonder whether anyone would have cared if the threat had been to brunette hair instead.)