Something straight ( edge of paper strip or similar to make the darker part,, or a plastic strip ( some of the folders had sliding plastic strips) pushed up and no longer protecting the cent. Not an error or mint caused problem, but a storage one. Do you know the history or assemble the set? often people put a coin in to fill a space and it may have received the pattern in a change drawer , or who knows. IMO, Jim
It might have been placed in one of those old "Coin Saver" cards that were so common back in the 50's & 60's. Chris