I'm guessing this one was malpositioned? There didn't seem to be much extra room in this shell. I waited a bit too long to help.
I'm trying to see which end of the egg is which, but if the air sack is on the side thats peeled away, it doesn't look malpositioned to me. Their heads are usually craned almost upside down and its beak looks like it's in the right place. Had it pipped?