Checked in recently at a CY and asked if I could get an upgrade. Was
told by FDC nothing was available. Was given a general room. As soon
as I got in room went on internet at Marriott reservations and found that
there were larger rooms and suites showing availability. This was about
4 PM. Checked several more times and still availability as late as 11:30 PM
that night. Next morning asked FDC about this and he said that Marriott
overbooks rooms and his room count will show negative (?) and that he
cannot upgrade rooms at checkin when he is holding those for confirmed
reservations. He says that Marriott reservation system allows them to
overbook because a lot of the time people don't show up.

I called Marriott reservations three different times and spoke to three
different people and they all say that Marriott does not overbook.

What has been the experience with others in this type of situation?