I had been defending Gaza from the Mongols as the English from many, many sieges. The worst of them, the Mongols breached the walls but I ended up routing them, slaying three family members, with judicious use of the ballista towers armed by a skeleton force of 14 men spread over 4 units. It was a heroic victory indeed!

I rebuilt the walls, and the garrison to a full stack, and even had a couple extra units of cavalry outside the city when the Mongols came back for another round. I decided to sally and wear them down a bit. That went well, I retreated back into the castle and tried to exit the battle.

It said I would lose if I did.

WTF? I went back to the battle, made sure that all my units were back inside the walls. Yep, everyone's fine, gate's shut? Hm. Try to exit again.

Still says you will lose.

It's late, I'm tired, I'm hoping it's just a gliltch, but when I exit, I do indeed lose and the vast majority of my garrison is killed, including nearly all of my archers. Gaza falls to the Mongols shortly thereafter and I've not been able to get it back.

It wasn't until the next morning I figured it out: I still had those two extra cavalry units as reinforcements. Since I hadn't lost any units in the battle for Gaza, they never appeared on the map, and they were still outside the castle.

I was so furious that I found what few men were left of those cavalry units and sent them directly into the Mongol Meatgrinder.