David Petraeus
Exposed by an FBI inquiry, a four-star Army General and Director of the CIA was reported to having exchanged vulgar emails with his mistress/biographer. The two might have gotten away with the affair, which was over by the time Petraeus was exposed, except that his mistress, Paula Broadwell, had sent threatening and anonymous emails to a longtime family friend of Petraeus, Jill Kelley. Kelley asked an FBI agent who was an acquaintance to look into the emails, which were traced to Broadwell, and then the email drafts were discovered.
