Well, you're over simplifying this process. The doctor's office often bears responsibility for these types of issues.
VERY high level, here's how it works:
1. Doctor's office fills out a claim, coding procedures, adding all the patient info, etc and submits it to the insurance company(ies).
2. Insurance company processes the claim and determines what's covered, deductibles, coinsurance, etc.
3. Insurance company pays doctor and notifies the individual what they owe.
The biggest point of breakage is #1. The doctor's office. If they fill out the wrong form, fill it out wrong, forget to do it, send it to the wrong place, use the wrong procedure codes, etc - then it's wrong. And this stuff is complicated, and keeps changing. The medical world just switched over from one set of codes to a much more detailed set of codes. Probably TONS of errors happening there.
Now, the insurance company absolutely can mess it up. The more complicated something is, the more likely an error is. So, if you've got two sets of insurance, it's harder to get it right. A claim for a week long hospital stay including surgery, tests, ICU, etc is way more complicated than a simple doctor visit for an ear infection.
Rantk81, the 3 examples that you list I see broken out like this:
1. In vs out of network providers - The doctor's office may have done the claim wrong. It's also possible that the listing of in vs out of network doctors at the insurance company is messy. Without further detail, this could go either way.
2. Procedure coding - the doctor's office almost certainly messed up. The insurance company may have contributed, but it's usually the provider side.
3. Coinsurance/deductibles - This one can go either way. When you have coinsurance, that just makes it harder in general so errors are more likely.
What you should be doing to help at least catch stuff:
Actually read all the EOBs. Match it to a doctor visit. If you can't or don't understand, call and ask. Fraud is a thing, so if nothing else you'll be helping to combat fraudulent insurance claims (which raise costs for everyone in the end).
If your provider has a billing person who sucks, you either deal or find a new provider. You could complain to the provider's office that they're always screwing up claims and please figure it out, but they may not do anything.