What's your sweetiest mistake in relationship?

I was in the process of a break-up (not together but stuck living with the guy for over a week until his mother flew him to her home)...and I got together with a man twice my age (which made him 40 at the time)...and he was married, with a daughter a year younger than me! I cannot regret it, because it helped me to realize and change many things about myself. He and I are still great friends and see each other often, we talk and share problems and whatnot, but I could not be "the other woman". He was definitely my favorite mistake.