Only time it happened for me other than taking a bump was on NW when the plane pushed back from the gate and came back after sitting for over a hour due to mechanical (it had allready been delayed at the gate as well). I was doing the trip as a mileage run, so since I was going to miss all my return flights, the agent rebooked me for future travel as a trip in vain. Still got the mileage for the first segment and that more than made up for the inconvenience .