I had this happen as well, but I was bumped. I flew from SAN to PHL on US and got there only to find that my flight from PHL to BOS was overbooked. The next flight was like an hour later so I offered to get bumped and was rebooked as Y on the later flight--got 150% mileage bonus and an extra half-segment. Nice little bonus on top of the free plane ticket for getting bumped.