Insurance errors
Check the date of service before you check anything else
Most coverage questions are answered by which day the service happened, not by which plan you have.
Before arguing about a bill, confirm three dates: the date of service, the date the claim was submitted, and the date the first statement arrived.
Your coverage on the date of service is the only date that determines who should have been billed. Coverage today is irrelevant to a bill from seven months ago.
The gap between service and statement
In our case a lab test was performed on March 2. The first paper notice we could find was dated August 15, about five and a half months later. That gap matters for two reasons: insurers have filing deadlines, and a very long delay is evidence that something went wrong on the provider's side.
A useful check
If the gap between your date of service and the first statement is more than a couple of months, ask why. A polite, factual question costs nothing and sometimes reframes the entire dispute.