There are two very common errors that affect most in this set, the first being cards that have missing hologram/foil on the back side; the second is much more rare(update 10/1/13, maybe not so much more rare, found another stack that was supposed to go with this post...), with a missing copyright on the back. From the first link above we see the cards that are missing the "Copyright 1990 Upper Deck Co. Printed in USA" run from cards #101-199, which is consistent to the cards that I found missing copyrights. I went through ~15,000 1990 Upper Deck before making this post pulling all versions of these that I could find. This is in no way indicative of all hoards of 1990 Upper Deck, just of the stock my LCS had in their backroom(most likely since 1990, hah). Of the boxes I found, one of the 5,000 count boxes had cards sorted numerically(five #1s, three #2s, four #3s, etc...), one box was sorted into four or five sets(cards #1-800), and the other had no rhyme or reason - completely random sorting(not numerical, not set, not by team/position/player...). Here's my findings:
No copyright: 104,106,108,111,2x114,2x115,116,117,2x123,128,3x157,164,166,2x167,2x168,169,170,171,173,177,181,191,193,199(Twin!)
Missing hologram/foil: 2x411,412,413,2x414,415,416,417(Twin!),418,419(Twin!),424,425,426,431,432,434,436(Twin),439,440(Twin!),452(Twin!),453,454(Twin!),455,457,458,2x-459,460,461,493(Twin!)
So you can see why I like this search, there were several Twins found missing stuffs. It's actually how I first found out about these, card #493 caught my attempt while sorting one day. Other than the missing section there is nothing that stands these numbers apart from any other in the set. The fronts look as they are supposed to, it doesn't look like someone took an eraser to the card, they hold up to viewing them in the light... Very odd. Also odd, all of the ones that are missing the hologram/foil were cards that are in the #400-499 range. Strange times these be. Have you found any of these? Lemme know in the comments below?