Coin Dealers: Describe a memorable mistake

Borrowing from SethChandler, who I think starts some of the most interesting threads, would the coin dealers describe a purchase that they had great confidence would sell quickly of for a sizable profit, but ended up being a bust (no pun, please, save it for another thread)? Perhaps we could all learn from such a mistake made by a professional.
Robert
Robert
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the ignoble memorable mistaketh.
Camelot
A few years ago I passed on another 1792 half disme. The dealer who had it did not specialize in that sort of thing and was looking to sell it to rainse money for a client. The price was around $8K. Not long after that the price zoomed to nearly $30K.
My 1866 Philly Mint Set
I'd say one of my bigger mistakes was when I called the mint about when they were planning to stop selling the Capitol Visitor Center $5 in unc. The guy told me there were only 800 coins left in inventory and after they were sold that was it. I figured out that the mintage would only be 5-6,000 pieces and it would be a great buy so I bought 20 pieces. Made good money on them but if I had some guts I would have bought 200 coins. If I did I would have made $200 each and that would have been $40,000. Opportunities like that don't come around often and I blew this one.
Interestingly enough someone who called the mint after me and figured it out bought the remainder of the mintage and made a ton of money. I think that person bought something like 600 pieces.