I harvested a couple of hills today and got quite the bounty. Many were large sized too......but they all had knobs on them & the knobs were almost as big as the main potato. I tried to take a pic, but it got lost in cyberspace. I created the thread because it is a fixable problem.
Apparently the knobs are formed when the potato is stressed....usually from too high of temps or temp fluctuation and/or infrequent watering. The knobs are still good eating.