Could someone choose to tax at 4 / city if they held both of these?
COINAGE: Why would you ever choose to have a tax revolt if you can prevent it?
Maybe you've made a deal last turn to give a player a city? Or you know one of your cities is going to be sacked by player X, so why not do it yourself, make player Y take your city, and then she'll have to give up a card when sacked instead of you? Edit: On second thought, since the beneficiary chooses, I suppose that second idea wouldn't really work.