Hospitals intentionally billed patients incorrectly

I have been billed to pay a copay for a preventive care procedure. According to my insurance contract, all preventive care is 100% covered. Had I not checked my medical bills, I would have paid on a bill that did not warrant a copay. Because it has happened to me on numerous occasion I believe this is a common practice for some major hospitals.. I think of others who might not be aware of their medical insurance contract, such as the elderly or others who might be ignorant of all of their medical benefits...