I had a hospital bill show up on my credit report from a city where I have never been about 7 or 8 years ago. The company that owns the hospital also owns their own collection agency. After explaining that I had never been in that city a hundred times, I demanded proof that I owed the bill. They sent me a computer printout with my SSN and the amount. That ain't proof. Show me a document with my signature on it. It was a battle for several years. It would get removed and then pop up. I refinanced my house in 2009, and I had to write a "letter of explanation" for the lender's file.
Finally, the statute of limitations expired. It popped up again one month later, at which point I threatened to sue them, certified mail, with copies to all the credit reporting agencies. It disappeared the next month and has not reappeared.
As I understand it, medical billing errors are the biggest problem in credit reports. I understand that credit scorers have reduced the weight given to unpaid medical accounts, especially when there is only one. Given the total lack of accountability of these collectors, that makes sense.