unkilldoo
Bench Warmer
junkwaxgems is a great informational website for errors of that era. I've been referring back to that site, for years. So much so that I have the site bookmarked.Mostly it was quality control. The set has also reached almost cult status for it's varieties and has gotten to the point where each card is heavily scrutinized. For example, some cards have a "missing period" variety in the third sentence on the back. Most of those don't really have a lot of collector appeal unless you REALLY like the set. There are a number of straightup errors and corrections (for example, one of the Joe Montana award winner cards has Jim Kelly's stats on the back...and there is a corrected version).
There are some really really popular varieties though. The two that come to mind are the Fred Marion card with John Taylor in the background with his belt unlocked (interesting visual result) and The Dexter Manley card that references his **** related suspension on the back (line removed really early in production).
Here's a good list: https://junkwaxgems.wordpress.com/2010/01/07/complete-1990-pro-set-error-variation-and-oddity-list/