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
Sold my Marsh amp & preamp after my divorce. Sold it for pennies on the dollar. It felt like therapy then. Now I just feel like a dumb ass. 
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 😖