Minor bug, you didn't check to make sure the city losing the population was greater than 1. You could be killing cities.
Haven't read all the replies past the first few, but figured i'll answer this since it happened to me. Forgive me if its been addressed.
So i planted a new city and i got pushed to -1 unhappiness. During the time it took to build a colusseum i lost a few pops. The newly built city also lost its one and only pop. Did the city disappear? Nope... it showed 0 citizens. Opening the city screen confirmed that no people were working any tiles. The city grew back to size 1 from the food the city tile produced.
What i'm wondering now is... what if the city with 0 pop lost another pop... would it go -1?