I spent the entire morning on the phone and various websites trying to sort out a medical bill. Insurance paid its portion and I paid mine months ago, but the hospital sent it to collections. They had applied my payment to the wrong account, then didn't rebill me. They say they sent a bill but I wouldn't not have recognized it because it was for a different amount. And their website bill pay system is way over complicated, yet doesn't keep track of paid bills.
This is not the first time I've caught them in billing mistakes. So my advice is to keep careful records. Cross-reference insurance EOB with all orginal bills. For more ways to navigate medical billing, read on I Think My Head is Going to Explode