Ah, but you forget the fun thing that can happen:
You go in for X. Insurance company defines X as a set of ICD-9 procedure codes for that health care organization, for that treatment for that indication. Not all providers will use all of those codes, usually some subset as the exact treatment may vary across providers under that health care organization. The health care organization instead sends over claims for those ICD-9 procedure codes, plus some other ones that your insurance company feels aren't appropriate for your treatment.
You then get an explanation of benefits from your insurance company, stating that they covered X, Y and Z, but A, B, C, D, E, F and G are your responsibility, because it exceeds the negotiated or "allowable rate" between the insurance company and that health care organization.
Or worse, the whole claim was merely coded incorrectly by the health care organization. Insurance company feels it's then not their obligation, and straight up denies the claim.
A through G is some amount of money that would absolutely crush the average person.
Your insurance company stands behind what they laid out in their statement of benefits for the claim, and won't budge beyond that, saying the rest of the costs are your responsibility. After all, they feel those procedures weren't relevant for the treatment, and it exceeds the negotiated allowance with that health care organization for that treatment. The health care organization's billing department is aggressive, and says since your insurance company won't pay, you need to.
TL;DR: "Balance billing."
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Here's a case where your child falling off a bicycle and needing stitches could cost you $19,000 if you don't lawyer up: https://wamu.org/story/20/11/25/a-kid-a-minor-bike-accident-...
Entirely unrelated to the specific scenario I laid out is ending up in an out-of-network emergency room. Again, easy to end up with a $20,000 hospital bill for falling off a bicycle because your insurance company says it's not their problem: https://www.vox.com/health-care/2019/1/24/18195686/vox-zucke...