What has been your costliest mistake in this hobby?

A Psvane KT88 with a lint short brought down my beloved VAC Phi 300.1, requiring that it be shipped to Sarasota, Fla. for repair. This happened prior to a national show, so VAC couldn't get to it for 3 months.  No music for 3 months. One of the most depressing periods in my life. Psvane no more and the VAC now has circuit protection.

Taking this hobby too far. Believing your satisfaction will come with buying the best of the best. Trying to reach the holy grail of sound with each purchase.
Not focusing more on the music than the equipment.

In my early days of diy modding fried out CJ MV-55 after upgrading all caps, still have the fried thing, someday...  Another early modding attempt building a mm only phono preamp, adding tubes and many circuit changes, adding step up transformers to the design, couldn't get it to fire up, still have this one as well. This one wasn't a great financial loss, but lots of labor time.  Think I know what's wrong here, so have future plans for resurrecting.

Diy  modding can be expensive early on, hopefully one learns. Haven't damaged any thing in the past ten years or so. I suppose I could rationalize these aren't losses as they've taught me what not to do. Still, I'd like to get both units up and operating just to see how they sound.

If this is a true story, and I doubt it.  I don't know how you can smile every evening at 6:30.  I honestly don't understand it. 
You admit in a public forum that you are a liar and in this case also a cheat.  It is one thing to say well I don't know if and how well they work, bad tubes, and the cost of any repair etc, I'll offer you $250 for the pair.  He can accept or reject.  Only looker or not, you knew what the were,  $30,000  + monoblocks (new) and you told the kid they were “organ amps” from an old Hammond electronic organ... not worth much."  Now he should have done his homework there is no excusing that but you lied and therefor cheated him.
I and most other "real" men who know who and what they are, including their flaws, work on their flaws to improve themselves and have character consider what you did disgusting!  A "real" man has character, honesty and integrity. They can be trusted.  You have none of those qualifications.  Maybe your wife is mad at herself for marrying someone who has no character.Karma has a way.....maybe some day you'll get home early and find your wife blo...g that 23 yr old kid.Have a nice day.

Bought a Signature 9 SE CD(and DVD) player for nearly $4,000 25 years ago. Used it for about 50 hours. Just sold it for $500 on Audiogon. It had much worse sound than my Kyocera 300 and 400 CD players from 1984 (sound like a Mac 30 amp, gorgeous mids, light bass, not overally detailed). It sounded sluggish and mediocre at the frequency ends. It had a difficult dual function remote, tough to use. Another big mistake was selling my Acoustat 2+2s and replacing them with Martin Logan Monolith IIIs. For the little money I got for the Acoustats, I should have kept them and just upgraded my system more (this was back in the 1990s).