Several of my regrets were lost to @Carthago; so, at least I know they found a good home! I recall an uncommonly-attractive, Balbus denarius, with undisclosed Leu 17 provenance, that particularly stung. I don't mind underbidding to other collectors - it means he/she wanted it a bit more than I did. I get most upset when I lose to a dealer, only to see the coin marked-up 100% a month or two later, because that means I misjudged the value of the coin.
Like JA, I have sooo many losses it would take weeks to list LOL But my most recent since it already exceeded my available budget:
LOL, I BET you are really "sorry"... VERY nice Denarius. Is that control mark 'G' for Mister G? (Hilarious flick)
That's a lovely example. They dont come up often and that one is of the finest style I have seen. Maybe it will come back around!
I made a bidding mistake and lost out of an AU-58 1921 US Half Dollar for $300. That was a $2500 coin.
I bid this one up & up & then up again this last weekend. I started the bidding on this coin a month ago & placed my final bid at closing. A competitor had the same high bid as me but he apparently placed his bid before I did. It was a fair auction & I just lost. I don't know if I should dislike the competitive bidder or apologize to the guy for making him pay more than necessary. Not many coins have a colt on the obverse & an owl on the reverse.
Unassuming coin but one I've been wanting, a little too much this time but I will get one eventually.