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?
Buying brand new what I thought was “ state of the art” only to find out 1-2 years later that was “out of date” and new changes had happened HDMI for instance and now my 6k gear was worth only $1k and couldn’t be upgraded. OUCH
I purchased Bob Carver M350 Black Raven amps and I thought they were great! I still do!
But, I thought that if the amps were so great then with his speakers together they must be a great combo. Wrong big time! The speakers required so much power that the amps meters were pegged, fuses blew, and they had a terrible sound. 8000hz and up was a disaster. Traded them in quickly for a big loss. 

Adding damping material tweak to my 15" woofers... when removing one of the woofers from the enclosure I dropped it resulting in the magnet moving and locking the voice coil... Slick move trying to get that final 0.001% improvement that killed a woofer that is no longer manufactured.  So after 6 months trying 3 pairs of woofers w/ similar thiel parameters I finally landed on a solution with a minor crossover change.  Note there is no damping material on the new woofers and I'm perfectly fine with that.  Ugg.
I bought a pair of Carver Silver 7 tube monoblock amps at an estate sale. Paid $250 for both.

Ive been offered $8,000. Each.

I still have them. But my wife reminds that I can sell them, every time we need quick cash.

i should never have let her see the text I got from the seller; the 23 year old kid who sold them to me at his grandfather’s estate sale. He still texts 3-4 times a year, asking if he can buy them back.

At the time I bought them, I was the only looker. I told him they were “organ amps” from an old Hammond electronic organ... not worth much.

So the kid is mad at me, my wife is mad at me.

I have a big smile on my face, every evening at 6:30 pm....