as dan posted , ive done it a cpl times whilst travelling the usa,

every time i just ring the airline withing 4 hrs of the flight and just say i wont be on the flight due to injury ,


in one circumstance on a us internal flight ( washington to ohio ) when i rang them they seemed amazed id done it our of curtosey that they offerd to reschedule my return foc ,


shows that a bit of effort on your own part sometimes goes a long way