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?
Actually now that I think about it the most costly mistake I made was CD. It came out right after college when I was broke and moving around and storing my Technics and records. Bought into Perfect Sound Forever. Only years later when I dug it out of storage and hooked it up did I realize my mistake. Still had kept some records, but gave most away. Listening to stupid CD BS cost me thousands.  

Made up for it since though, let me tell you!
I had a Superphon DM220 amp on the floor. Ataxia was starting to make it difficult to balance bending over so I was pushing on the rocker power switch on/off with my toe. Missed one time and pushed the power LED in.  The amp became a welder and popped both woofers in the sacrifice.
Ended up with better woofers and a 400s amp.
Total ~$600 damage.
Hooked up an Audio Research D200 amplifier to a pair of low Ohm Electrostatic Speakers. Damage to both pieces. Definitely learned an expensive lession.
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.