After playing a game in which my opponent played Princess, I noticed that the costs of most of the cards used in that game were 2 coins less than the correct cost on the "Select Kingdom Cards" menu. Presumably the effect of Princess was incorrectly carried on beyond the end of the game. I tried playing another game with those cards, and the costs were correct again, so I don't think this bug can affect gameplay. I don't know if the other cost-reducing cards (Bridge, Highway, Bridge Troll) or events (Ferry) can cause the same bug.
Thanks, solved.