Originally Posted by
shack
A "lean" state in a diesel will have no bad implications whatsoever, in fact it's the best possible state for them to run.
Rich however is another issue altogether, I have a feeling you are thinking of a petrol engine.
A melted piston top is a sign of overfilling, if it is only one, it is probably a dodgy injector.
Some rotary injector pumps can start to overfuel as they get older/wear.
In one of your pictures though the damage looks to be collision damage not heat.
If the lash caps haven't been replaced, it's quite possible one broke, then got pushed down by the rocker and released the collet on that valve.
Have you found them all?
If they are all ok, it could also have been a failure caused by the rocker being massively out of adjustment.
You may also never know what caused it.