I had a pen full of quail and one hen laid eggs without shells, only one. I'm not sure what caused the problem, but I do know that once a chicken eats an egg they are hooked. I had 4 chickens and the only way I'd get an egg was to catch her laying it and grab it fast. I finally just ate the chickens and now I'm starting over with a differant breed.
If you can find the one that is laying the "shell free" eggs, I think I'd seperate her from the others before they accidentally get a taste for raw eggs.

That is a good idea to take out the chicken just in case they break one and start eating them.
I did see a friend who has 80 chickens and asked him the same question and he said that sometimes a chicken will lay eggs too soon before they are mature and the first dozen eggs or so will not have shells. He said it was common and that in two weeks or so that she should be having egg shells too since they have the oyster shells available at all times. My birds are Delawares and I will defindately eat the culprit if she doesn't staighten up in a two weeks. They are duel purpose birds. They are so tame, they are like those chickens on the TV trying to disguise themselves as foster farms chickens. THey always beg for junk food like pasta and table scaps like tv dinner corn and anything we dont eat that is organic or vegetables. I dont give them any meat of any kind. ( Mad cow disease should teach us we dont feed cows ground up bones and meat for protien based feed to get big, fat cows for meat) anyway they (chickens) begg for food like cats and are always ;under my feet when I am out in the yard. BIll C

That makes sense about them being too young, I hadn't noticed the shell less quail eggs lately.
I bought 25 of the White Cross meat chickens and they are just fat, lazy and friendly. I didn't keep a light on them at night so they wouldn't grow too fast, still a couple had problems with their legs.
Also I got fertile eggs from my neighbors and have an interesting group of mixed up chickens who are everything from Buffs to Barred Rocks, Bantams to Brahmas and every combination in between.
I think chickens may be omnivores. When I was a kid I always had chickens and when we killed a snake the chickens found it and it was gone in a few minutes.
Cows are strickly vegans, or should be. I haven't eaten "cow" since 1999. The only meat I eat now is what I raise myself. I know where it's been and what it ate.
Oops, better get off my "soap box" before I get going on commercial food production.
Good luck with the eggs.

One of our two-year-old hens has gone from laying eggs with thin shells to no shells to absolutely no shells - just a puddle in the nesting box. We give the hens plenty of oyster shell. I just read this on one site:

"The cause here may be low calcium/phosphorus/vitamin D intake or imbalance, or salt in the drinking water. It is more common in older birds but can be a temporary fault in pullets."

On the other hand it's been raining for two weeks so although our hens are outdoors most of the day, maybe it's the vitamin D - maybe some hens have trouble synthesizing it?

"Shell-less eggs are held together by the membranes alone. This may be related to a lack of calcium or vitamin D. Hens synthesize vitamin D from sunlight so this will not be a problem if they are free-range."

We had chickens eating their eggs. We were unable to get to them before they were eaten by the chickens. We learned that their egg laying boxes should be made much smaller and darker with the opening for them to get in only about 7-8 inches tall. The egg eating stopped immediately.