Students browsing the science sections in their library may naturally assume that all of the books are scientifically valid and accurate. Science collections may also contain books that may not now be accurate, either because they are out-of-date, or never belonged there. While out-of-date knowledge can sometimes be beneficial, invalid books can only mislead. The well-known book Worlds in Collision by Immanuel Velikovsky is a case study on how librarians can handle such books. For these books, an explanatory note can be placed on the book's online catalog entry or even in the book. The book can also be weeded from the collection or reclassified. Each one of these putative treatments has potential advantages and disadvantages.