What has been your costliest mistake in this hobby?

Mounting a brand new cartridge. Had a small screw driver laying on the platter. Moved the arm over to see the cart alignment. Screw driver slammed into cartridge and knocked the stylus off!
 Stupid mistake. I now have a small tool box for mounting carts and everything has been checked for magnetization.  

Had a pair of B&W 805D that then diamond tweeter had shattered.

bought the replacement for $900.00 decided to change out myself. After taking the damaged one out, I set it on the work bench. I opened the box with the new one took it out and set it on the work bench. The magnets pulled them both together and smashed the now one.costing me $900.00

I purchased a pair of Von Schweikert VR33 speakers used for the retail price of new because there was a 6-9 month build time and I had to have them. After I received them I realized they sounded terrible in my room. I tried all the usual fixes like moving them, putting pillows in spots where I thought things sounded better, etc.

A year + later, my wife mentioned I hadn’t listened to the stereo in many months and only a few times since I got the VS speakers.  I Vacillated another 6 months before I admitted to myself that I hated the speakers. I then bought into the Golden Ear line and I am extremely happy. To finish my story, I sold the Von Schweikert speakers for 66% less than what I paid for them, and also lost my shirt on shipping.

My most costly mistake...like most, is knowing this hobby we all love so much will never end it's cost....sad but true...

Bought a pair of Higher Fidelity 138 monoblocks for a sweet deal. Very very powerful. But needed some love. Postponed that. Bought my first biamp cable, a vd hul Revelation. It had the most thick spades ever made. The cables were also very heavy so it tugged on the speaker terminal, and there wasn't any good connection with it too begin with due to thick spade

 1 day before Christmas 2018 I was blasting music, cables came loose and a ball of smoke erupted from my amps.  I really loved those van de Hull wires so kept them, happened again to the other monoblock 😭

Maybe 8 months later both amps worked better then ever: rebiased, new lv matched transistors. Didn't like the highs coming out the speakers sold them. But in hindsight it was a system mismatch

The guy who bought them couldn't believe his luck. 

Repairs: €760 down the drain