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
35 years and lots of mistakes.

1) Selling good gear for more expensive gear that was no better in the end or worse.  
2)Trusting dealers opinions. 
3) Buying unproven designs that sound great then break down... 
4) (Just recently) buying from 2 designers that went bankrupt (or died, Roy Johnson Green Mountain Audio) while they have my money and gear under repair or upgrade and I never get it back. This alone has cost me 20K. Several other members here got the same treatment.


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.....:-)