I had one of those the other day, I upped the calcium with some yogurt. Unfortunately my hen hasn't laid again yet so I'll have to wait and see if it fixed the problem.
The hen that laid it hadn't laid in a little over a month.
Mine lays an egg like this every day. Sometime she skips a day. But she is pretty regular. These eggs are really large too.
I do give them calcium, I give them crushed oyster shell. We give them bananna when they get to the stage that no one will eat them, apples, pears. They also have a pretty large pen, so they sort of free range. Their pen is 75 feet long and 12 feet wide.
She always lays it in the nest too.
This could have been caused by anything from a "hiccup" in the process to a vitamin/mineral imbalance all the way up to a physical defect. Here's what you can do. First make sure the hen is getting a well balanced, complete diet. A quality layer feed and free choice oyster shell is a good start. You can then monitor the eggs for 12 weeks. If the eggs still look that way, try increasing the fresh greens, protein, and calcium a bit more and monitor another 12 weeks. If the eggs still look that way, decide to accept that they may always look like that or cull the hen. The bright side is, they are probably perfectly normal on the inside!
I feed them a 20 percent layer pellet, I rotate them outside their pen with my other birds. The oyster shell is where they can get it when they want it. I have only had her a couple months. Thing is, I have 6 of them and I don't know which on it is yet, with out sperating them. all look fine, all have clean bottoms. I just get an egg like this just about every day. I give them shelled corn, and scratch as treats. The only thing I can figure is that she is just going to lay a weird egg. Which ever one it is, she has been laying an egg like this the whole time I have had her. So I wouldn't figure it is hiccups. They are not messed with, other than when I go out to take care of them. I just don't know.