1991 Pro Set is not quite the variation wonderland that 1990 is, but there are a handful of tough error cards throughout the set worth spotlighting. One of the few known varieties from series 2, affects two Indianapolis Colts players: #523 Chip Banks and #529 Mike Prior, both can be found with or without their text or back.
Initially upon release, I recall finding a handful of these, however, they seem to have dried up as they now show up only once or twice a year and usually command a high premium. Here is an example of the text-less variation:
Junk Wax Gems 
Hi. How can I purchase these from you?
1990 Pro Set Photo Contest #790 Steelers with all-3 Pro Set logo color types (purple, blue and black) – $3
1990 Pro Set Photo Contest #793 Ronnie Lott with all-3 Pro Set logo color types (purple, blue and black) – $3
1990 Pro Set Photo Contest #795 49ers with all-3 Pro Set logo color types (purple, blue and black) – $3
Hi Terry,
Unfortunately they recently sold. I have been out of town so I hadn’t updated. If I locate any more, I will list here so keep checking back. Instructions on how to order are found at the top of the “errors for sale” page.
Thanks for checking out the blog!