Many years ago (back in "Royal Medallion" days and before the CRJs), I was confirmed on the last flight of the night from ATL to LEX. I wanted to be on the earlier flight but it was "sold out." Once my in-bound got to ATL, I went over to the gate for the earlier flight to LEX to see if I could get on it. As I got there, the GA was just arriving and there was no one sitting in the boarding area. I ask the GA if there were any seats open and he replied that ALL THE SEATS WERE OPEN. Some how, the Delta computer system had marked the flight as fully booked even though there was no one on it. In the end, another PAX who had the same idea I did (standby) and I were the only PAX on the MD88. The FAs moved us up to First and the other PAX and I did the safety briefing to the FAs. We were both such FFers that we knew it by heart (and still do). Booze and snacks flowed freely for the flight.