I suspected my doctor trying to pass the costs of an interpreter posting the charges on my credit card without my knowledge. My health insurance, BC/BS, requires only a $30 co-payment which I paid at the office, given a receipt. I noticed a $150 charge on my credit card statement, and immediately call the credit card via VP to dispute the charge. They assume it's a billing error, I think not. I faxed them a copy of the receipt to them and during investigation resolved the case in my favor.