I should've been clearer about the doctors. Whether a doctor is part of your insurance network can change unexpectedly when the contract/relationship between your doctor and the insurance company changes, even if you yourself don't change insurance. And this happens relatively often (annually and sometimes more). So you can get a surprising bill if you go back to the same doctor.

The databases that maintain this information are not always accurate or up-to-date. So the doctor might say one thing, the insurance company another, and both be wrong in subtle ways. Nobody knows what's going on. It's often a crapshoot what you'll owe after a simple doctor's visit, even if you've asked all the right questions and stick with the same doctors.