I got one the other day from one of my hens. They are 13 months old. Not the first time that it has happened. We usually get a good laugh and then I toss them into the compost.
That is so cool! I wish mine would give me a warning like that. I was watching my Red Star today and she finally got into the nest box I made, and then...she sat down!! But she didn't lay anything. It was like she was trying it out just for kicks.
I've lost count of the number of tiny eggs that I have gotten over the years. Many of them even smaller than the one you posted. I have opened every one and have never gotten one with a yolk in it.