That is one you do not see to often. Can not tell you how it happen unless there is a metallurgy problem with the retainer or the valve stem to allow them to wear. Save to replace both = no rework. LOL
I agree they are both getting replaced. Now the question is do we play nice guy and lap the valves while we are at it. I guess I will have to lap at least the exhaust valve since we are putting a new one in there.