Services done in January '07. Dentist billed Guardian who was not covering me. Guardian paid bill. November '08, Guardian wanted money back because they shouldn't have paid. Lincoln, who was covering me back in '07, wasn't going to pay because of 15 month grace period was past. Dentist has paid back Guardian and is going to bill me. What are my rights in this? I believe that I gave the correct information to the dentist.

If it's an ERISA plan you probably are out of luck with the carrier. But if it was the dentist who erred, as then it would be his (or his staff's) error that caused the loss, you may split the loss and keep the dentist or play hard ball and take the position that you need not pay him. I doubt he'd sue you. If he does or sends it to collection, you could dispute claim and defend.