Actually, to begin with, you are wrong: if you bought a gallon of milk last week and were not, due to an error or oversight, asked for money for it at the time, the store *can* ask you for the money now--the law is very clear that a failure to charge at the time does not invalidate a debt. (Obviously, for the cost of a gallon of milk, it's not worth it for the store to do anything as a practical matter, but the legal principal remains.)If two fillings were done and you were only charged for one due to some error or oversight on the dentist's part, you still owe for the 2nd filling you received. If you don't pay them, they would have the legal right to sue you or put you into collections.