Billing for services not rendered
Many times unscrupulous doctors will actually bill the insurance company for services that didn't actually take place. They depend on patient ignorance and the advantage of the insurance companies not being there in the room at the time of treatment in order to get away with this one. That's why it is important that you as a patient look over your bills or have someone you trust go with you and then examine the itemization. If not, your insurance company could be getting hit hard, and they will pass that on to you in the form of increased premiums.
Misrepresentation of the goods and services rendered
Sometimes the bad guys won't flat out lie about what they did, but they will misrepresent what the goods and services rendered were used for, and again, it results in an overcharge. You don't want to cause a rift between yourself and your doctor, especially if you trust the care that he provides, but if you start to suspect that his office is trying to "pull one over on you," then you need to do something about it. Contact your insurance representative at once and voice your concerns.
Misrepresentation of the nature of the patient's illness
Sometimes a doctor may present a patient's illness as less (or more) serious than it really is. This can trigger liability under the False Claims Act as well because many times an insurance company will not cover something that is unnecessary for restoring the health of the patient. If a doctor seriously thinks testing or sending to an outside specialist is necessary, then there is no issue, but they have to be able to back that up by showing a past treatment pattern that indicates an escalation.
When dealing with the health care world, you as a patient must stay on your toes, and do your very best to protect yourself from overcharging and fraudulent practice.