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
In about 2000 I sold my entire system after my daughter was born and gave my sound room in our house at the time for her bedroom. I had Vandersteen 2CE's Audible Illusions Pre Calif audio labs DAC and transport and sold it all for Rotel surround receiver and energy surround speaker system. That lasted about 6 months. I miss tubes
Accidentally bumping the stylus on my Lyra Delos cartridge and knocking it off....

ended up trading it in for credit and purchasing Lyra Kleos
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.