What has been your costliest mistake in this hobby?

I haven't had any fires or explosions, My worst mistake was more an error of judgement.

I had a large Vandersteen speaker w/integrated subs (dual/opposed 12" woofers per speaker) & required an external speaker amp. I was using my first serious SS speaker amp, an Adcom 555 that I had modified, greatly improving the depth & impact of the bass. The Adcom was an astounding bass amp.

But I read somewhere that Perreaux amps were the very best for subwoofers--even better than Brystons. So I paid ~600 for a used 200 WPC Perreaux. I was so certain it would work that I sold the Adcom to a pal (for not much $$). The Perreaux proved a distinct step down, audibly inferior to the Adcom. That was 32 years ago. My friend still uses the Adcom every day.

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...