Biggest Card Regret?

I'm sure all of us have one or two cards that we have traded or sold that we wish we had held onto.  I have been selling cards on Ebay since 2006 or so and have been selling at shows for the past year.  There is one card that sticks out in my mind as one that I really wish I had not parted with.  Of course, I got a good bit of money for it (it sold for around $100 on Ebay), but that's not the point.

I pulled this awesome Buck O'Neil sweet spot auto from 2007 Sweet Spot Classic while I was still working at the card shop.  Of all the cards I have sold on ebay (I keep the scans on my computer), this one is the one I regret selling the most.  I'm not sure what Upper Deck was doing when the auto was mounted on the card, but it has a definite character of its own. 

I hope to add Buck O'Neil's auto to my collection again (on a baseball).  I would prefer to add him to my HOF collection, that is, if the Hall ever puts him in.  O'Neil's omission from the Hall is definitely the subject of much debate.

Do any of you have that one card that you wish you had not traded or sold, and what would it be??

Hidden in my Collection - 2007 Sweet Spot Classic Roberto Clemente

I got this card back in 2007 when I could actually spend some money on packs of cards. Yeah, I know, that seems to be a recurring theme around here... I USED to be able to afford nice things. I remember being stoked when I pulled this card. I love the old flannel jersey. This is the only relic card I have of Clemente and I have no desire to ever part with it.
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I just watched the movie 'Chasing 3,000' the other day, which was primarily set around Clemente's 3,000th hit and 2 brothers' quest to make it to that game. It got pretty sappy at the end, but it was a pretty good movie nonetheless.
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I'm a Clemente fan, for sure. He was a true ambassador for the game and is still missed, almost 40 years after he died. I'll post more about Clemente himself when I get to his RC in my 1955 set.