Rebellion due to unhappyness. I played a self-imposed ICC game, I think I was doing a cultural victory. Japan took a lead and ended up conquering most of the world, I had been the only one not to fight Japan. With victory something like 30 turns away and with a tech lead and lots of money I declared war on Japan and starting buying Mechanized Infantry every turn. I steam rolled about half of Japan's empire racking up lots of unhappyness. Then Japan came with a surrender, offering every single city except his new capital. I accepted and suddenly I had more unhappyness then I'd ever seen, enough to start making rebel anti-air units appear around my capital. I was able to deal with the rebels and proceed with Utopia. With one turn until victory the peace treaty ran out and I declared war on Japan again, taking his captial in one turn with the force I had waiting on his border.