I had this happen on my KDX200 a few years ago. Stuck at full throttle while in top gear riding at the river! It had quite a bit of work done to motor too. I'd hate to have it happen on a 500. I held the kill switch in but it took a long time to stop it. Turned out the throttle cable was about to snap, there was some small bits of frayed cable that had fallen off and got stuck beside the slide.