lets say you start removing half a thyroid and determine during the case that the whole thyroid should be removed and at the same time a major nerve is affected by the removal of a cancer and that nerve will have to be partially excised and then repaired.

You just turned a 2 hour case into a many hour case and you can only bill for the specific CPT codes that you perform. Its that simple. You can't build in a risk premium for everyone and tell the insurance company to pay you more for the occasionally higher risk patient. They don't do that. Neither will a single payer. It just doesn't happen. so if a patient requires a fixed price (not allow to change) then what you will find is the doctor will close the patient and tell them they need to go back another day for the rest of the surgery.

this is just one scenario of many that could come up because the practice of medicine is fluid.