Years ago, out at sand mountain, i had a buddy, fairly new rider, on a old XT550? i think. He was messing around on this little track around the bathrooms. He crashed, we went to see what had happened. He said the back tire locked up. THen we noticed a nice big puddle of oil under the bike. Turns out, the upper chain roller(above the chain), fell off, somehow got under the chain, wedged between the chain and front sprocket, and ripped the cases in half. He never liked that bike anyways.