Originally Posted by Andrew R Stewart

This is my first thought. Shifting under pressure. The rider will almost always say they didn't. Are there scratches on the chain stay underside near the rings? If I saw the tell tale signs of this on a bike coming into the shop I'd bristle at a warranty claim but likely put one through anyway. But I would also be willing to be honest with the warranty guy over the phone. Something like "we think it was from jam shifting but can't say for sure. Any consideration you can offer will help us keep our customer happy and devoted to your brand". Likely both the brand and the shop will take a hit. Andy.