What has been your costliest mistake in this hobby?

Biggest?- easy:
Had Old Wurlitzer 78 rpm jukebox, noise & static, reached in, wiggled the plug where it went in the amp to get better connection, got my finger & thumb around the speaker field coil output, most definitely got my attention. Lived, so not most costly.

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.