The Zulu were really unhappy (like, -100something happiness) because of their stubborn autocratic leader, and one of their biggest cities flipped to my empire (I was Venice, I think). I didn't want the city and I was pretty afraid of the Zulu (they were my allies, but also by far the biggest military in the game), so I tried to sell it back to them, and were confused that they weren't willing to give me anything for it. I ended up giving it to them as a gesture of good faith, and then the city flipped back to me next turn. I ended up puppeting it because I didn't have the heart to raze it. Imagine how that would be.
"We've succeed the revolution, we are under Venetian control now. Now, new commander, what do you want to do with the city?"
"Raise it down!"
I couldn't do that morally, so I just took the happiness hit. Hopefully giving it back for one turn gave me a positive diplo modifyer with the zulu. They didn't declare war on me for the rest of that game, so who knows. I did have about 20 trade routes with them, but that really was probably decreasing their money loss from about -1500 to -1400, so that didn't actually help them much. Their military and leader stubbornness was actually that big.