I negotiated a deal over the phone with a dealer down in OKC. We negotiated exact terms and exact price, before going down there. Showed up and surprise ... my calculated payment didn't match their calculated payment. They brought out a finance contract showing the correct terms, but an inflated payment, due to hidden fees which we thoroughly discussed would not happen. When I called them out on it, the salesperson "went back to the financial officer to check", came back and was like "yeah we made a mistake" and the hidden crap was removed. If I had not calculated the payment, even though they had the correct terms on the sheet, we would have been stuck paying that payment for the life of the deal simply because I agreed to the payment and the hidden fees buried within.
It's not enough to just agree on an interest rate and duration. You have to calculate the exact payment to make sure they are not sneaking something in there in fine print.