I really don't know, I was kind of surprised when the mechanic told me. He told me that it can happen from heavy peddling stress when shifting, which I thought seemed strange, because I figured it would take a LOT to bend a cassette. I haven't had any bad crashed on the bike, nothing. The biggest drops I have taken it off of was like 2 feet, so I'm just so baffled as to how I bent a cassette.