It has happened at least twice to me - I sell one-offs and do not sell OS - so that is not the problem. One was from an item sold last August, which somehow was relisted and resold again this year and it was so embarrasing when I coudn't find it. But going back through my sales records, discovered why - it had already been sold. And then it happened again recently with an item originally sold last November but somehow got relisted and then resold.