It all looks so neat in the grocery store - cartons of eggs in small, medium, large, and extra large. You might think they come from similarly sized chickens: Petite-sized hens laying cute little eggs. Plus-sized matronly hens laying jumbo-sized eggs.
Not so, back at the coop.
Chickens are pretty lousy at quality control. A hen will give you a dozen beautiful large eggs, then pop out a half-sized runt. But hey, everyone's entitled to an off day. Sometimes even a way-off, really bad day, when a shell just sounds like too much work and the hen says hell with it, a thin membrane is gonna have to do it. And of course, a membrane doesn't do it, and your fingers poke right into the goopy center. For obvious reasons, I don't have a picture of those. But the photo below shows eggs from standard-sized hens of the same age. The two on the right would go in the "small" carton. The two on the left are medium to large. The one at the top center is extra large. And the one at the bottom center...well, as I'm sure the chicken said, "Ouch!"