I've experienced the city give away "bug" in the two games I played. I didn't even know it was a bug. It was pretty funny though how I can pretty much always start a war, take a city, and then negotiate peace to get the rest of their cities. I've come to expect it now. And all I've have a is catapult, a couple of archers, and a couple of swordsmen, and the AI will give me 5 cities when he could've at least attempted to build units to defend while I try to take each city one by one.