BobC, I'd think the Magie error is a critically important one for the hobby. I'm guessing the 1989 Ripken FF and 1990 Thomas NNOF are definitely in the mix, along with the other ones affected in the same Topps printing sheet. Doyle is super-valuable, but maybe Nodgrass variation is not in the top 50.
What about cards made of people who were not licensed to be printed and had to be cut down or removed from sets, like 1959 Fleer "Ted Signs for '59" or 2006 Topps Alex Gordon cutout or 2007 Lavarr Arrington (FB) cutout.
[URL="https://www.comc.com/Cards,sr,=2006+alex+gordon,+297,ot,i100"]
Ted signs was pulled because Fleer didn't have permission to include Bucky Harris on cards, and is therefore a valued short print I would put on a Top 50 list.
1959 Fleer Ted Williams - [Base] #68 - Jan. 23, 1959- Ted Signs for 1959 [PSA 5 EX]
Courtesy of COMC.com
Also on the modern list should be the RC of Beltran, even though it wasn't corrected.
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1995 Topps Traded & Rookies - [Base] #18T - Carlos Beltran (Juan LeBron pictured)
Courtesy of COMC.com