well, evolutionists say that chickens are the decendents of dinosaurs, so maybe she de-evolved a little in the laying process... put it in the bator and see what hatches!!
I also found this, from The Poultry Forum, though I don't know how old your hen is:
"Sometimes in an older hen, things don't work quite as well, the egg can spend a little long in the egg shell gland, causing extra calcium to be laid and a slightly ridged texture."