In the discussion of gun policy around the recent school shootings, I've encountered a few gun control opponents pointing to the Multiple Victim Public Shootings paper by John Lott and William Landes, which argues that having civilians carrying guns in public prevents multiple-murder killings.
It's an interesting thesis but a little googling reveals that Lott is a bullshit artist. Salon has a quick overview of the reasons to be skeptical of Lott, with several links, as does the Washington Post, Media Matters, another fact checker, Mother Jones, and Wikipedia. (Media Matters has a bunch of articles about Lott, since he turns up on TV and other media a lot. I found a critique of one of his books particularly instructive. A couple of pieces about his TV appearances provide good examples of his deceitful moves.)
Another Washington Post article surveys the literature on Lott's “more guns, less crime” hypothesis and find that the evidence is against it. Tim Lambert at ScienceBlogs has two articles on evidence that he has cooked his statistics. He even has used a sock puppet identity in online discussion fora to defend his own work.