My wife had a minor surgery a few years ago and the hospital told me that the bill would take a while to arrive because they need to "fight" with the insurance but they assured me that everything was backed up by the doctors.

When the first bill (from the insurance) a few items were denied like the extra hour of hospital bed because she wasn't feeling well after the anesthesia. The hospital called me and told me not to worry and they would ask for the doctor who denied it and since it wasn't a doctor they would approve it after a few weeks

At the end it was still expensive but nothing was denied