I once wrote an email to a coworker complaining at length about the inadequacies of our boss. My critique was withering; I pulled no punches. As I reached for my mouse to click send, I noticed the email was addressed to... our boss! Yes, I was one mouse click away from a VERY embarrassing moment.
My mother would say the moral of this story is if you can't say something nice about someone, don't say anything at all. Obviously, I didn't listen to my mother. The moral I draw is check twice before you click send, because once you do, you can't get it back.