I cracked open our first bad egg the other day. I had found it when I was raking the litter in the coop. Anytime I find an egg like that I put a big X on it so I'll know it is suspect. And don't worry, I never give these eggs away.
This egg looked like it had incubated about a week based on the development inside. And since the weather has been cooler lately, it must have incubated several weeks ago when it was hotter.