I got one of them once. I have eleven hens and they've been laying for about five months at roughly eight eggs a day, that puts it at... a lot. I've only gotten one thin shelled egg and that was three months ago. I don't think it's anything to worry about. My birds have a steady diet of layer pellets, a decent brand as well, but you never know what's going to come down the chute, so to speak. I've gotten eggs with what appears to be calcium deposits as well.
Not too long ago I had a hen that was laying eggs without shells. I brought her inside and had to bring a food bowl and water up to her and that was the only way she would eat or drink. After a couple days I brought her back outside and she started eating like she was insane to get more food. The eggs quickly turned hard again and have not had a problem since. I guess she was just a bit sick and didn't feel like eating. Bringing her into a warm area seemed to be just what she needed. She also was deprived from eating grass while she was inside, maybe that got her used to eating the chicken food again????