This has definitely happened to me however sadly it hasn't happened recently. I've had Delta cancel a flight and put me on United. I always give the FF # to the gate agent and ensure it is on the boarding pass for the flight I do indeed fly. I've never had the mileage post automatically with the carrier I booked but was unable to fly HOWEVER I've always been able to get the credit through a quick call (or email). I wish this would happen more often, DOUBLE BASE MILES!!!