I had something similar happen. New model, on its 4th flight with an AR500 receiver and an X9303. I had it idling in the pits. I looked away for a second, and the next thing I know the engine has revved up and the plane is in the air. Fun moment! The throttle stick was at idle, the battery was new and freshly charged, and the radio was in my hand. It's a brand new engine that was being well-behaved after a quirky break-in period.