Mermaids: The Body Found

Mermaids: The Body Found, is a 2012 docufiction created by Animal Planet. It convinced many people that mermaids are real and that organizations, such as the NOAA, were covering up the evidence. It is based on the Aquatic ape theory. At the start of the film a message is shown that says it is speculative fiction. Still, it was so convincing that people ignored this. NOAA thoroughly debunked this show as being false and distorting scientific evidence to support a speculation.

This is hardly the first time people took a piece of fiction a tad too seriously. Famous examples include The Bible, the Quran, and Orson Welles' 1938 "The War of the Worlds" broadcast (although the latter has been greatly exaggerated). Or, more recently, the 1995 film Alien Autopsy: (Fact or Fiction?) which caused many to believe in the whole "aliens at Roswell!!!" nonsense.

There's also a 2013 follow-up: Mermaids: The New Evidence.