We've had this happen 4 times in 4 years. Once, the site we reserved was occupied, the campground simply screwed up and gave us a different site. The owner came over later, very apologetic, and gave us a bottle of wine.
One time the people didn't realize it was their day to leave (?) and were a couple hours away doing something. The park employees gave us a different site we really didn't like, but it was only for 1night.
Another time, at a very nice Texas gulf resort in Port Aransas, the people were taking their granddaughter to the Houston airport and we had to wait over 2 hours after check in time.
The 4th time was almost nothing.
JK