What battles have you been completely confindent of winning, then suprised as the AI uses a trick to send your soldiers mass routing, or a fluke makes them brake through you lines and flank your best soldiers.

My story is when i was playing as England, i launched an attack on Scotland, the numbers were pretty even, but i was counting on my general to stop my men from routing and using flanked attacks to demolish the enemy. I rushed my spearmen in too quickly and forgot about the enemies reinforcements so sent my cavalry to kill them, but my spearmen were almost dead by the time my cavalry arrived and the enemies reserve spearmen came in and slaughtered my cavalry, Paul, Prince Roberts son, died along with the majority of his men. The main purpose of this battle was to lower my army count as my money was close to -4000, (my army was large and my economy sucked)

The Ransom amount was close to 2000 gold, but still, i managed to kill Edmund of Canmore, and King Phillip of Scotland So it wasnt all bad.

This was probably the worst, if you dont count the slaughter of Christendom at Jerusalem, many good men died there...