I suffer defeats from time to time but they're generally close. I can remember of only one ashaming defeat.

I was playing as the Julii waring against the Gauls (how original ), trying to win a settlement from them (in Spain). I was in a rush-rush period, winning 2 regions a turn, thus I couldn't replenish my troops all the time. I recruited 4 mercenary warband groups, setting my stack number to a grand total of 800 men (4 hastati's, some archers, only one cavalry unit alone with my general...).
The Gauls had 7 barbarian cavalry units a few archers and 4 warband units.
I put my mercenaries on the flanks to keep a strong core in the center.
The battle took place in a plain, summer.

I stayed on the defensive. In a blink of an eye, they launched a full cavalry charge, straight in the center (in my hastatis), 3 of my hastati units broke at once. I tried to flank their cavalry with my mercenaries but their infantry struck them down (some even routed at the sight of the enemy...). In less than a minute it was a massive rout and my general fled.
Henceforth I don't use warband units