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
Oops. Forgot to mention...sold my Maggie 1.6s and my Panerai PAM 001 after my divorce. Not even a year later, I bought used 1.7s and a Panerai 8Day. Go figure. 
I wanted to replace my Oppo 95 Blu-ray player with a new one.  A friend of mine insisted that I get the 105D model even though the 205 was  just introduced.
I bought a used 105 ($950) for almost the price of the new 205 $1250) which I now understand is a better machine.  The 205 is now selling for $5999 on Ebay!
What a stupid mistake! 
Just bough 5 pair, (yes 5) of Quad ESL 57s in solid cosmetic condition but with various needs. Not sure what I got myself into but I know for certain the $$ faucet will be wide open on these. Sending them up to the “Quad guy” in NH for the breakdown 😖
Too lazy to unload the gear I upgraded from.  Lots of cables, reclockers, power supplies, tube pre amps, phone pre amps, music servers a CD player I used for a month.  I have big plastic bins full of stuff.   My better half is telling me to get rid of all the cables after she went looking for an ethernet cord for her laptop.  Good thing she only looked in the bins labeled cables.