I just had a pewter die on me in the egg right before it was ready to pip. My son did a dissection on it and nothing appeared to be wrong other than a strange small black lump below its heart that discolored the tissue around it. I couldn't tell you what it was or even if it had something to do with its death.
Sometimes they just die on you for no apparent reason, then again sometimes healthy grown snakes will do that as well. Being that this is a pewter, I wonder if this is more common with morphs than it is with normal snakes?