The $1 Jame Madison??? (That is one ugly looking portrait). I try to only pay face value for all my coins so I rarely have any regrets. I don't really have anything that stands out very much right now.
I have some sort of George Washington commemorative medal from the 70s that was a waste of $5 and takes up space. I also have a silver casino chip that has no real value and probably isn't even worth the bullion value.
A George III Bank of England Dollar that only grades AU-55 sold to me as MS-63. Win some and lose some, but it has taught me to be much more careful so the toll was a learning experience.
great question, I usually think of the ones that got away. I have to think about this one, since I like most of mine, especially since they have gone up in price. (prior to that, I think a 1909 S Lincoln, slightly overpaid, but now it is a clear profit.)
All of them! Its a burden that I have to carry. Seriously though, probably my first 10 or so early comms. I didn't really know how to grade at the time and bought them as UNC. Some ended up being AU and but many would grade less than that. Bought them all off of ebay. I do not buy unslabbed coins anymore on ebay. An expensive lesson to learn.
A 1944 Walker Half Dollar that was sold to me graded as MS-60 that came back as "Improperly cleaned" Luckily, I didn't pay alot for it.....got it when I first got back into collecting and needed to learn stuff...still do but getting there. Smaugy.