I was promised a year's road tax £145 as part of the deal when I bought my leon. The smarmy salesman then denied promising the tax, my mrs shouted at him in front of a showroom "you are a liar."
A common ruse is they agree a trade in price for your car, then when the day comes to do the deal they come up with some reason why they can't give you that amount and offer you less. That has happened to a few people I know. One guy I worked with just stood up and said he was leaving. They tried to up the offer but he said he wanted what they initially said or they could stick it and they caved.
I walked away from a Leon FR last week. The price of the car was ok but the trade in price for our car was poor and I just felt the dealer wasn't making any effort. It was actually the owner of the dealership I was dealing with, everyone else was on furlough. The kerbing on the wheels and bad stone chips had not been fixed, the cigarette lighter cover was missing and there were other bits that were not prepped to the standard you would expect on car of that value. I also knew that the previous owner had chipped the car and it had a towbar. I wanted a longer warranty in light of the question marks over how the car might have been used but he didn't offer to include one, we would have to pay for it. He wouldn't give me money off for no trade in, basically he wasn't giving us anything at all so I didn't buy it.
It's as much the principal as anything. If dealers think they can treat you like that and you'll still buy the car then they'll continue to do it.