I came across a Clayton Kershaw card a while back that I had no idea even existed. I originally thought it was unknown to me because of how short-printed it was, but after a bit of research I discovered its rarity stemmed from gimmick.
If you look closely you'll see "The National 2012" stamped on the right side. Much like Expo 1/1s in hockey, I guess Panini took extras of the card and made them National-only pickups. While it is gimmicky, it's also super-duper short-printed.
As it only has a print-run of 5, I was interested. I guess the funky nature of the card scared away bidders, as I landed it for just 99 cents plus shipping. Numbered 3/5, it'll find a good home in my Kid K PC.