There have been a few; such as frying the finals in my SB-104A, buying used gear that turned out to be dogs. I once bought a Kenwood R599 receiver that some idiot had tuned the 10 Meter band for CB ..... but I managed to rectify that.
Without a doubt, the stupidest thing I have ever done, in my own mind, was to sell my Heathkit HW-8. Back in the 90s, I donated it to one of the Piscataway Amateur Radio Club auctions. If memory serves me, John WA2F was the winning bidder. He's also a member of the Raritan Valley Radio Club. One of these meetings I should ask him if he still has it and make him an offer. Chances are it probably went via the RVRC Hamfest Club table many moons ago.
I've sold a lot of equipment that I've no longer wanted; but when you sell something that you've built with your own two hands ..... there's something regrettable about that. I wasn't into QRP 100% back then. I sure wish I had it now, even though it's definitely not the greatest QRP radio in the world.
Yeah, I could go on eBay or QRZ or eHam and hunt out another; but it wouldn't be the same.
72 de Larry W2LJ
QRP - When you care to send the very lest!