I have just found on one of my Yukon Gold potato bushes - a stem coming out from one branch with 5 marble sized green tomatoes growing. Is this common as I have never seen or heard of it before. I have taken pictures.
This is the fruit and I see them every year on my potato plants. I find it depends how long you leave them in the ground before digging up; the longer in the ground, the better the chance that the plant will develop fruit. And being green they are not always easy to spot.