Happens to my SO constantly. We went in for a urology visit, scheduled for months beforehand by the urologist, get a test done, yadda yaddda yadda. Now, up to 3 years later, we still get bills for that particular visit (in addition to other follow-ups) on a monthly basis. It's ~4/8 hours per week of extra work nearly every week in dealing with this nonsense. It's as if they don't even bother with sending us a bill anymore, they just go straight to collections companies. At this point, I know better than to think it's ever going to be fixed. The insurance and Hospital billing phone numbers are literally on speed dial.

On particular phone call stands out. We got our insurance on a 3-way call with my SO and the hospital billing. For a urology test they charged us ~$800 for the test strip (just a single one!), which the hospital tried to claim was not covered (nothing, it seems, is ever 'covered'). Even the insurance person on the phone was aghast at the hospital: "Wait, so you are telling us that even urological tests, like a basic pH strip, at the specific office for urology, is $3000? What is our client even paying for, like, what is it exactly, that a urologist does? How much does a smile cost?"

US Medical billing makes Kafka blush.

That's part of why I have Kaiser, too. I don't need to worry about providers of any sort being out of network (nurses, assistants, docs, labs, etc.)

They'll tell you the exact opposite once you get your bill and they're trying to collect. I just had that conversation with a provider myself, after I got the bill.