The plant will sprout from a small potato left in the soil. New potatoes will form from this and the old one from which the sprout originated will decompose. You can consider the new potatoes safe to eat, which if you had planted new seed, the outcome would be the same. However, my experience with left over seed potatoes is that those plants will not produce as well as new seed. You could eat left over potatoes if you dig them up now, but leave them for an additional harvest.