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
Was installing a brand new Benz LP Moving Coil cartridge, my first ever high end cartridge and was wearing a long sleeve shirt, well, you know what happened. I snagged the cartridge tip somehow with my shirt, and after a few expletives, saw what I had done...the tip was canted sideways. That was a bad day. 
Marantz DV9500 (B-stock): bad drawer mechanism, went tits-up after a couple of months, it was deemed unfixable; Marantz SA8260: ditto, plus background hiss and noise; Audio Research CA50: bad power supply caused massive pops at random during music, not disclosed by Agon seller who shall remain nameless. All three were not cheap and ended up being turkeys. I suffered a loss on all three.
selling my trl gt 400s, gtp preamp, and soundlab a1s. still chasing that lost Magic! rip P.W.
My costliest mistake was to buy nearly one thousand vinyl LP's over the decades that had only one song on the album that I liked.

The 'one' song per album I liked cumulatively proved very expensive.