I passed on a PSA 4 1956 Mantle for about a 1/4 of what they go for today but my biggest regret to this day is not acting sooner with my Uncle's 52 Topps cards, that included an Eddie Mathews.
At the time, I told him if he was ever going to sell them, to let me know, but not knowing back then what I know now, I should have hounded him for sure as he ended up selling them to a pawn shop sometime between then and 5 yrs ago.
I'm not sure why he did that, other than him thinking he likely could get more at a pawn shop than he would from me, but to this day, whenever I see a 52 Topps Mathews, I immediately think back to what could have been.