I don't know if this is more common than I think. Someone gave me this hen and her sister three years ago. They laid eggs for some weeks and stopped afterwards. I reckon they were around three years old when i got them from a friend of my mother in law ( they couldn't keep them anymore although my suspicion is that the hens were laying fewer eggs and the owners didn't want to kill them).
Anyway, fast forward three years, yesterday afternoon and today morning this hen didn't come for food ( they are free range). I went to the coop, looking for her. When I saw her, I thought she might be dead but to my surprise, I saw an egg. She was covering three quarters of it with her body.
This afternoon, when I went to check out, she was fully covering it, so I remove a feather a little bit and took the photo. Then, I covered back.
Is this more common than I think, or because most hens are killed after stopping laying eggs, there's not enough data.
To be honest, I am ecstatic 😀