While working on something tangentially related, I've come across several "conservative" books, blog posts, and articles asserting that H.L. Mencken was a eugenicist. The argument seems to be as follows: evolution is bad, and leads to eugenics; Mencken was a leading secularist and defender of evolution, as with his reporting from the "Scopes trial"; ergo, Mencken must have been a eugenicist. (It doesn't help, I suppose, that Mencken was an elitist and something of a misanthrope.)
In fact, Mencken was one of the leading, and one of the few (!) prominent opponents of eugenics in its heyday. His American Mercury published one of the first major scientific critiques of eugenics. And Mencken consistently rejected eugenics in his own writings.