Oh, I've got one!
Playing as Brittany, I slowly managed to conquer France by about 1480, supplemented by my Native American land grab. I was in the middle of putting down a couple rebellions when I had to leave the computer and take care of other business. The lesson I learned this day? Save.
I came back to my game 10 minutes later to find I was in 5 separate wars, 3 of which involved multiple major contenders. I was at war with England, Burgundy, Castille, the HRE, both the Mexican and American natives, and about a dozen minors. Not to mention the war exhaustion-triggered rebels... all over the place. I would have been able to fight against any one of these powers individually, but as a group I was pretty much outmatched, especially since a third of my country was occupied by the time I saw what happened.
Rage quit!