• bad David Petraeus’ alleged affair with author Paula Broadwell, which occurred after he went to the CIA, was uncovered after officials had figured out that both Broadwell and Petraeus were using private email accounts to communicate with one another. While the accounts used pseudonyms, Broadwell came under scrutiny because of threatening emails she sent to Jill Kelley, a Tampa volunteer she believed to be a romantic rival. While there was an investigation of Broadwell due to her having classified documents, they ultimately found she obtained them legally. 
• worse Apparently, the FBI agent who tipped off the agency to the matter was taken off the case because … wait for it … he got obsessed with Kelley and sent shirtless photos of himself to her, which is an incredibly stupid idea for any FBI agent to do. The agent, who was a friend of Kelley, did tip off a member of Congress to the case, which (after other FBI agents found the tie between the person sending the emails and the person she was emailing) led to its logical conclusion — Petraeus’ resignation. source

EDIT: And here’s why Petraeus created a security risk by using Gmail in this manner.