What has been your costliest mistake in this hobby?

That is a really good question!

I’ve been fortunate. No bad choices, just some that might have been better in hindsight had I been better educated at the time. Biggest lesson learned over time is perhaps to always do your homework regarding what things are most likely to work well together and be diligent to manage the financial risk associated with trying costly new items. Avoid snap decisions based on limited information.

two

buying a cool used piece that no one could fix when it failed (carver passive preamp)

very expensive cables

My only costly mishap was about 30 years ago when my dbx dynamic range expander took out a woofer in my JBL L96
Guess I had too much bass boost. 

I've rewired my room, whole panels, component mods, all kinds of stuff going back to building a Dynaco ST200 in high school. Trying hard to think, there must have been something accidentally dropped, or scratched, or burned out somewhere along the line. Sorry, but I'm drawing a blank.  
There was a made in China Jungson amp that turned out to be a total piece of defective crap, two reps from China even came out to see first hand the noises it made, lied to my face and then refused to honor their warranty. But my dealer stepped up so even the royal mistake of buying made in China didn't cost me anything but grief and frustration. 

Honestly the one bonehead mistake I made was putting a 0.8A Orange Fuse in the Melody. The value on the factory fuse was barely legible, so that was a factor, but how in the world was I so out of it as to think a 50WPC amp can run on less than one amp? Dope. So it blew. Instantly.  

Then of course I realized what happened. Told the dealer. And surprise, they replaced it anyway. So sorry, no sad stories here. Just happy endings: Buy American. Buy Synergistic. And they lived happily ever after.

I always shut down to change a rig. Biggest mistake was putting an opamp into EE DAC wrong orientation.  Not a major dollar crisis. I triple check all connections,  so no kabooms.

Dealt with two liars over the years, an that cost me a few hundred.