Example 1: Fred writes Wilma an email containing some nasty thoughts about Jane. This is considered gossip and should be avoided anyway; however, in this case the email somehow gets to Jane (e.g., Wilma passes it on either on purpose or accidently). This can seriously damage both Fred and Jane's reputation.
I've been in the information security business for many years and it's wise to assume that there's no such thing as perfect security. I was told once to not put anything in an email that you'd regret it going public and it seems like a good rule of thumb for many reasons.