What has been your costliest mistake in this hobby?

Selling gear I should have hung onto!

waiting so long to buy PS Audio !

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