Strange

Ok my friend that had 636 had a wreck the other night and almost got ran over by a truck. The strangest thing happened. he was riding infront of us and all the sudden he did like this halfway flip. the bike went forward and sideways. I came really close to hitting him and his bike. he is ok just some road rash. we were looking at his bike and his chain was like locked up and tangled. It was weird what could of caused this, or was this just one of those things that heppends?

Look at the hardware on the axle adjusters, make sure there wasn't a failure there. That usually will only happen if adjusters were loose or overtightened to the point of failing, or the chain has some serious stretch with loose and tight spots (you'd be hearing it if that was the case)

Something went seriously wrong there...don't see that happen much.