What has been your costliest mistake in this hobby?


For example :I recently learned a hard lesson- I accidentally ran voltage thru my $3000 MC cartridge (kiseki purple heart).  I have a TT with 5 prong connector and a phono cable with a 5 prong connector.  I accidentally swapped where they plugged into and ran electric thru the tonearm into the cartridge.  It was a stupid - not thinking- hasty mistake. When I corrected the problem the cartridge was fried.  An avalanche of four letter words followed!

So what has been your biggest and/or costliest mistake?
polkalover
My costliest mistake, not buying a copy of Donald Fagen’s, ‘Nightfly’ vinyl album (Mobile Fidelity’s, 1step Process). At the time that it was available, it cost $125. Now, it’s out of circulation. You can still get a copy, but they’re going for $400. Ugh, I could kick myself for not getting it when it was available! What the hell was I thinking???
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Buying excellent components in the first place based on the recommendations of others before I had developed the expertise to really evaluate it, then selling it for better (?) stuff to 'improve' my system.  I knew nothing (to speak of) about synergy or set up. There is quite a bit of stuff I wish I still had which is now considered quality vintage, is hard to find, and is quite expensive. Go figure.....:-)
Buying a power amp on reputation alone. Bought an Audio Research GS150 thinking it had to be great. It was ok but hated the non responding power meters. I'd sell it in a heart beat if I could.