19th-century plagiarists exposed by modern technology. And a more recent mathematical biographer's outraged response to the discovery of his own plagiarism. Via mefi.
On the subject of mathematical plagiarism, I was intrigued to run across two more such stories on Wikipedia recently: Dănuţ Marcu, repeated and unapologetic copier of whole articles. And, in contrast, a sideline to the Poincaré conjecture credit controversy concerning two authors who lifted a single lemma from two other authors' online manuscript, and promptly apologized.