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
Bought a Krell HTS 7.1 for $8000 MSRP. Dropped in value like an anvil out the window. Sold for only $800. OUCH! Technology sensitive products drop way faster than the market for High End stuff.
1) not listening before buying
2) not properly packing before shipping
3) not figuring out synergy between amplifiers/speakers
4) not considering room’s impact on sound

I'm sorry, you said just one.
  

Ham fisted and connecting arm cable to a high end Dynavector and pushed the connecting pin into the cartridge.
Bought a pair of Wilson WATT3/Puppy2 new in the mid-nineties that I thought was the last word, but eventually took an 80% loss at resale for a better speaker at less than half the price.  Ouch! 
Biggest?- easy:
Had Old Wurlitzer 78 rpm jukebox, noise & static, reached in, wiggled the plug where it went in the amp to get better connection, got my finger & thumb around the speaker field coil output, most definitely got my attention. Lived, so not most costly.