That had to be strange opening that one. I’ve seen a few things but not that.
The hen’s internal egg making factory has a lot of parts. Occasionally there are glitches, especially when they first start laying. Sometimes it takes a pullet a while to get all the bugs out of her programming, but those weird eggs can happen anytime. As long as it is not consistent in an older hen it’s generally not a problem.
The last part of the egg making process is in the shell gland where the hen puts the shell on the egg. That process takes many hours. After the shell is on, she then adds the brown color to the outside of the egg if she is a brown egg laying hen. If you open a brown egg and remove the membrane from the inside of the shell, you’ll see that it is white. Or you can sandpaper the brown off and leave a white shell. Normally the inside of the shell is pure white but some hens tint that a little. Still, if it is a brown egg the difference should be pretty plain.
I don’t know what happened there, but the hen apparently started the first shell. Before it was totally finished with the brown applied to the outside of the first shell, she started a second shell and finished it. Why would she do that? I don’t have a clue. Sometimes stuff just happens.