Open margin crown. What rights do I have?

I would let the dentist that did the crown know of the open margin. This actually happened to me and my prior patient wanted her current dentist to fix it. I took a look at the x-ray proof and refunded her insurance portion as well as her portion in full so she didn't have to deal with the insurance side of things.The prior answer is completely accurate in that everyone has a bad day now and then! If your past dentist doesn't want to do a refund, see if there's an agreeable solution.If not, and you have insurance, you can work through them for a resolution. If you do not have insurance, possibly a small claims court? I'm sure that would become a hassle that most dentists would want to avoid. 

Sorry to hear that you are having a problem with your crown. Everybody has a bad day now and then. I am sure that your dentist would rather make you happy by giving you a refund if that is your request, than to see you leave angry. If you ask with the right attitude I'm sure your old dentist will be very apologetic and do the right thing.