Same situation - received a single bill, which I believed to be in error because I had paid each time I visited the office for physical therapy over the course of about 6 weeks. My wife actually used to work at this therapy clinic, so I asked her about it and she said I should wait to see my explanation of benefits from my insurer and wait to see if I receive another bill the next month.

Well, I forgot that my EOBs are no longer mailed to me, and instead I have to download them. I never received another bill, and forgot about the issue. Then 6 months later I received a collections notice from a law firm. I really wanted to call and give the therapy office a piece of my mind, but I decided not to since my wife still has friends there. I don't think I ever saw an impact on my credit score, but as two91hokies mentioned, perhaps there is some secondary record that doesn't necessarily show up in your score.