The one that sticks in my throat is the time I decided to max bid in a hot auction and ended up setting a record price for a boxing card. Not the sort of record I want to set ever again. Happened just before the Great Recession and the card fell off the end of the price charts never to recover. I grew more and more disgusted with the card over time until I decided to just get rid of it. One of the other collectors I'd been slugging it out with and I sat down at a National and traded mistakes from that auction; he'd topped all on several lots and we decided to trade mistakes since neither of us could sell the cards except at extreme losses. Whenever I think of that card I throw up a little in my mouth.
At least I learned my lesson and mostly sold into the COVID run-up. I only bought a few minor cards for too much (under a grand, total). Oh and one expensive blue chip card that for resale, but that one has real potential, so I don't class it as a mistake. Yet.
A happier question is the most I ever paid for a card and was happy that I did. The one that readily comes to mind is this:
1972 STP is basically the T206 of NASCAR (in importance), and Petty is its Wagner, so having a signed Petty makes me smile every time I look at it.
Oddly, I also think of this one:
I had one for a long time, reluctantly sold it to a persistent buyer who was willing to overpay for it, almost immediately regretted it, and finally replaced it. I cannot explain why, I just think the card is cool.