Series 2 seems to be littered with pack-pulled wrong backs. These don't typically qualify for master set inclusion but some like to add them. I beleive they only exist in one combo for each player: Rob Blake, for example, will only be found with Donnelly on back and visa-versa. So one could essentially make a complete wrong back set. Maybe.
What is interesting about them is that unlike most Topps wrong backs from the junk era, 1990 Pro Set S2 wrong backs can be occasionally found in poly packs.
It is also worth noting that 1990 Pro Set FB S2 packs can be found containing wrong backs that have Pro Set FB player on one side and an NBA Hoops player on the other. They were heavily reported in hobby magazines back in 1991.
Owner and creator of JunkWaxGems, online source for unlisted error, variation and oddball info from the junk wax era.