What caused this odd egg?

[Cyprus]

My birds are currently on Agrimaster mixed flock feed, with oyster shells mixed into the feed. Scratch feed is given occasionally as a treat.
Some of the hens in this flock have not laid in a very long time.

What would have caused an egg to come out like this?

I have no idea who laid it. Whether it was a consistently laying hen or not is beyond me.

 

[Chickiemom25]

looks like the calcium didn't get laid down properly. I don't worry when it is one egg, only when it is consistent. Maybe she tried to drop 2 eggs in a day,

 

[Ol Grey Mare]

looks like the calcium didn't get laid down properly. I don't worry when it is one egg, only when it is consistent.

X2... Also you noted that many of your birds had not been laying recently. Often these anomalies occur when birds are returning to production and all the moving parts of reproductive system have not re engaged fully yet

 

[Cyprus]

Thanks everyone!