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 
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