Here is an old post from the Library of Congress Blog about a type inquiry that can confound librarians. It seems that over the years the LC reference staff has gotten “a surprising number of questions from people looking for several books that … are widely and definitively known not to exist.”
Tops on the list is “the President’s Book of Secrets which “a fictional invention for the plot of the movie” National Treasure: Book of Secrets, Regardless, readers still inquiry about the book in hopes of checking it out. The post also discusses some other examples of non-existent tiles that have been requested by readers who thought they were real.
One suspects that the folks at LC are not alone and that some of you can also cite examples!