Reason I think it had pipped rather than just got cracked somehow is because there was a straightish line around the middle of the egg and that's what I read happens when it pips.
"Pipping" refers to a single small poke through the shell (generally on the large end) so the chick can get outside air, then a while later the chick starts to unzip and hatch (a while can be 6 hours or 24+, they need to learn to use their lungs fully and practice breathing before they leave the shell, also ALL blood is absorbed from the shell membrain before they emerge).
It doesn't look like a crack, but a bit jagged along the edges resulting from many little pecks to open the shell.
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