Today's xkcd discusses the problem of "citogenesis," the problem where bogus information is inserted into Wikipedia, sloppy writers copy their information from Wikipedia, and then a cite to the sloppy writer is added to Wikipedia, making the bogus information "true." I've seen this first-hand.
Inspired by Amity Shlaes's The Forgotten Man, I was reading up on the American Liberty League, a bipartisan organization opposed to the New Deal's infringements on economic liberty; interested in conspiracy theories, I wrote in 2009 about the so-called Business Plot, an alleged planned coup in 1934.
I was curious whether there was any basis for the 21st-century riff on the 1930s conspiracy theory that Prescott Bush, the father and grandfather of presidents, was a part of the American Liberty League and the Business Plot. (The claim was that FDR cut a secret deal, undocumented by any history books, to avoid prosecuting the conspirators in exchange for their support of the New Deal; the claim is plainly bogus, as the American Liberty League continued to oppose the New Deal and went on to successfully strike down the National Industrial Recovery Act a year later.)
The only academic source for Bush's role in the American Liberty League I could find was in a Charles Beard book, with a 2008 introduction written by University of Massachusetts history professor Clyde Barrow. This introduction was almost word-for-word taken from an inaccurate 2007 version of a Wikipedia article written by a conspiracy theorist. I asked Professor Barrow where he got the source for his claims, and why the Wikipedia article was so similar to his later-written introduction, and got no response.
Moral is: don't trust Wikipedia. Or liberal history professors.