I have a couple.
My grandmother dated my grandfather for quite a while. He broke up with her and ended up dating her sister! He realized that it was really my grandmother that he wanted, and they ended up married until my grandfather passed away.
My most recent ex and I dated for three months in high school, but broke up when he went off to college (I still had a year left in high school) in part because of commitment issues, and I also think GIGS played a role. He came back about 6 months later. I had already moved on and was seeing someone else, but once that fizzled my ex and I reunited. We maintained a relationship for another 4 years, and recently split because the distance between us (we attend schools in different states) became too difficult and we also decided we should learn to be less dependent on one another and live apart. We left the door open for a future when I move back and if we are both single. Maybe I'll be adding that story one day!