I had a similar thing happen to me several months ago. I got a bill from one of my doctors for a visit I had in early 2007. I paid it, but wrote on the invoice that if they can't bill me in a more timely manner in the future that I will take my business elsewhere. I explained that there is no way by now I could even remember what the visit was about and whether this was a clerical error or not. They never wrote back and apologized, but I felt better for having let them know that I DID notice how long ago the visit was.