It's hard to make mistakes in card collecting. I suppose you could overpay or overtrade for a card. Maybe you prospect on a rookie player and he winds up being a bust so the cards don't go up in value like you'd hoped. More common perhaps is buying or trading for a card you didn't know you already have. I've done that for sure. But none of those things are really that devastating to a collector who's in it just for the fun of it.
One of my biggest mistakes in collecting has to do with the Topps basketball sets from 2000-01 and 2001-02. I found them at Rainbow, a LCS, for a really decent price. The catch was, they didn't contain the draft pick rookies. No big deal, I thought, at that price I can pick them up separately and still get a good deal. Yeah, that's what I thought. I figured Topps was Topps and their flagship cards would be in abundance. Nope. Fifteen years later, I still can't find some of them. Those sets must of been severely short-printed or something. Here's one I picked up on eBay recently.
This is the 2001-02 Topps of Kedrick Brown. The search started way back then with 18 rookies needed for the 2000-01 set and 27 needed for the 2001-02 set.
I still need the following from 2000-01. Any helpers out there?
#128 Marcus Fizer
#135 Jerome Moiso
#138 Mateen Cleaves
#145 Donnell Harvey
#148 Erick Barkley
#280 Hanno Mottola
I still need the following from 2001-02. Help please!
#225 Jason Richardson
#242 Jeryl Sasser
#245 Samuel Dalembert
#248 Trenton Hassell