that is a top habbit of diesel's ( i just call them dusels atm for some reason ) bit unusual on petrols unless there g60's
the cause of the fail was the big 19mm bolt not been tight enough. sadly officaly the cranck is now unservicable AKA fked
altho it can be botched if you get a new key and clean the keyway right out till its shiney push the key right up to the leading edge ( good one ) and pack the gap on the other side with some really good epoxy. let it go proper hard and then sligthly roughen the shaft and the inside of the lower cam belt pully cover both with plenty of strong loctite then torque it right up with a new bolt.