I know how you feel. Sometimes there's a little side-tributary to the root that is not found the first time. It is not uncommon to have to re-treat a root canal, and no, you cannot typically get the original dentist to do the re-treat for free. It's probably not negligence and if it were, it would be nearly impossible to prove. There are no guarantees that a root canal will be successful and your original dentist probably went over this with you when you had yours done. I was very reluctant to get my root canal retreated, but I went to an outstanding endodontist and wish I had done it earlier! It was not bad at all; she really knew what she was doing. She did, however, tell me that retreats have about a 66% chance of success. If it's not successful, then you have to get the tooth pulled. Just pulling the tooth w/out a retreat is much cheaper, but then you have to ask, what am I going to do without that tooth? Having an implant to replace it is much more expensive than a root canal. My redone root canal has been very successful and I am happy I did it, even though it was around $1700 and insurance only paid less than half of it. When my root canals were originally done, I was told that I have very twisty roots, so it is hard to make sure that they get everything. When the endodontist redid it, she found the tiny side tributary that was feeding the infection in the gum, between two teeth. Good luck!