I had this happen twice out of several hundred rentals over the past few years.

Once at Enterprise, I reserved a full size and got a 2 door sub compact of sorts. I complained to the desk and they said that was all they had, I complained to the desk manager, and he said he can't do anything. I asked for his manager and he called some guy and put me on the phone with him. That guy, the GM, told the staff to comp the entire rental. ^

Once at Avis I had a full size, and it smelled of smoke, so they gave me a standard instead, and a $25 coupon for my trouble. Full Size and Standard are usually the same price (I still can't figure that one out).

Both times I feel like they went above and beyond.