Not an error and not done by Topps. It was done by a previous owner of the card probably a young collector. The Seaver picture on the back is a 1969 Topps Decal that has been rubbed off onto the back of the 1969 Topps Seaver card. The decals were inserts in 1969 Topps wax packs. Many kids rubbed the decals to the backs or fronts of cards. Just like they sometimes attached the 1961 and 1962 stamp inserts to cards.
