On Wednesday, I got one brown egg and one white egg. Ditto on Thursday. On Friday, I got one shell-less egg. I hopped over to My Pet Chicken for a diagnosis. http://www.mypetchicken.com/backyard-chickens/chicken-help/Why-did-my-chicken-lay-a-shell-less-egg-H85.aspx
So... As far as possible problems go:
I just put more salt in the water softener on Monday. But it's now Friday, so I kinda doubt it.
The dog found animal poop right outside of the coop this morning. So there was a possible stress factor in the environment last night.
One of the leghorns is "henpecked" right now, as evidenced by a loss of feathers on the back of her neck. So she could be evidencing stress now.
As far as possible positives go:
While I have two leghorns and only one red star chicken, I have only been getting one white egg for every two brown ones from my star chicken. Perhaps this shell-less egg is actually the indicator that one of the leghorns was a late bloomer and is only now starting to lay eggs.