First up are three 1989 cards that I was surprised to learn I did not yet own.
While I knew I didn't already have the Fleer All-Star card and I would have guessed that I didn't have the yellow Hoops parallel, I would have bet my entire Stockton collection that I already owned the white Hoops card. How I ever considered myself a decent Stockton collector without that card in my collection is beyond me. But, thankfully, the problem has been corrected thanks to the Diamond King.
With dull, small print on the backs of cards back in the day, it's no wonder I never turned them over. Also, I never collected sets, so I didn't care what the card numbers were, which also contributed to my lack of interest in the backs of cards.