One wiseguy suggested the screwdriver in the pic got sucked in.
Since this was brought to light, I have heard it happened to a 60's Triumph and a '96 BMW twin. Steve Hamel has put a Terry Prince valve in and declared the guide OK. The piece of valve rim broke off when he was tapping the stem through the collar. The break was across a 1/8" square section! I was yards away from a lot more money to repair things!
I had the front exh valve out and it was in very good shape, so I just have to whip the heads back on and ride it.
I am convinced that the valve had a flaw and caused the failure, the colors of piston, plug, etc. all looked good.