What would cause this to the valve?

[CO_Rider]

Was checking the valves on a '11 with 50 hours on it and found 1 of the exhaust valve stems chewed up. Shim has some marks on it also. Bucket seems fine. Any ideas on what happened?

Edited June 1, 2012 by CO_Rider

[SWIGIN]

Valve float?

If the valve floated the shim could loosen from the bucket and land on the stem cocked.