Ouch! That's probably the worst one yet.3) Nuking a weak neighbor strictly for the comic relief only to provoke the rest of the world into dog-piling me.
Once I was going for Diplomatic victory, and I kept coming up short on votes. Once, for no particular reason, I abstained rather than voting for myself, and that was the one time I could have won ! After that, it was back to me not having enough support, and I think I eventually got a Time victory instead.