This has happened often enough that it can't be a fluke. A regular doctor visit is supposed to have a $25 co-pay. I made a regular doctor visit last December. A bill just showed up in the mail. The insurance didn't cover $10 of the doctor's fee. The doctor now wants another $10.

What will happen, what always happens, is that I'll call the insurance and yell at them and they'll pay the $10. They make this sufficiently painful that it's almost easier to just pay the $10. This makes me suspect it's a deliberate business strategy. My employer negotiated for a certain set of rates and co-pays and heaven knows what else, and this is how the insurance company tries to make extra cash. Very annoying and probably makes them liable to a class action lawsuit, if only somebody were sufficiently annoyed as to bother suing them.