One of my blunders was putting my king and his only surviving son in the same army and then having that army destroyed in a siege. This happened when playing as England and crusading in Spain, all was going well and I was master of all of France and large chunks of HRE. Then all of a sudden the game ends when the king and his heir dies, it was sudden and totally unexpected. In one stroke, one short battle, I lost the game which was irritating, considering that I at that time had more armies and a stable economy and all was going well. I have played TW games for perhaps 8 years now and this defeat is remains one of the worst..