I'v elost the occasional battle, in rare (too rare) cases even to some impressive tactics on the part of the AI, but lost campaigns seem next to impossible. Every one in a while an early blunder(especially in some of the better mods) allows the comp to gain a huge advantage, but if I tough it out and keep playing they always squander it eventually.

I did quit a Teutonic Order campaign(my first) that was going very badly, but I don't know what would have happened if I'd kept it up.