i was 'walked' by the Hilton Mission Bay 2 yrs ago. I arrived at 1:30 am . I had garanteed the stay to my Am EX card. I had called them at 11pm to confirm that I WOULD be coming. Got there no room. They sent me ( did not provide transportation , unlike Starwood Lurkers comment) over to the Hyatt. Unfortuneatly the Hyatt was also fully booked and pu tme in a dump that used to be amotel that I think they normally used for storage - no heat or food available. It was awful!!!!!!
When I returned to the Hilton the next morning they did however put be in an oceanfront suite for the rest of my sty, send a letter of apology and fruit basket to the room and . they paid for the room at the Hyatt and did not charge me for that night.
It isnt fun - because you are tired and cranky and just want to sleep but it does happen
HTH