If you keep chickens for any length of time, once in awhile you'll come across a shell-less egg.
Take this one, for example. Looks pretty ordinary, doesn't it?
But it has no shell, only the inside membrane keeping it together.
The membrane is actually surprisingly tough. It hardens up a bit when exposed to air, and this egg was sitting in the coop all night.
The egg inside is perfectly fine. I just tore the membrane open.
It's fun to have a little variety once in awhile.