I've had the same experience, with a young Australorp, (brown egg-layer). This bird is a bit of a trickster, which is why she's named "Joker", I just never know what surprise I'll find in her nest box. She's my smallest hen, yet has laid eggs so large, that I've gone back over her to check her for prolapse..How does a hen this small, lay these huuuuge huge eggs?
And then there was the day she left a super-sized - white - egg. My guess is it's a double-yolked egg. I've put it into a lime solution, for use later this winter. There are no other chicken owners in my urban neighborhood, and my coop remains locked until I get home from work, each afternoon, no way could any other bird have visited. It's a mystery to me, why this happened, but it did, and only once, so far.
