I lost once because I was not one of the civs up for the vote. It was an OCC game and I was well on my way to a 20k cultural victory. I timed a war one turn before the UN was completed by a rival civ and they could not risk the first UN vote. However, by the time the second UN vote came up, the world was in two alliances, 4 vs. 3 and the UN owner was on the side of the four. I just got to a point where I could not hold enough civs on my side of the war. Probably could have done some things differently diplomatically that would have prevented the loss, but by the time I realized the bind I was in, it was too late. The most frustrating thing is that I was less than 20 turns from that 20k cultural victory.