What has been your costliest mistake in this hobby?

 My biggest mistake was trading in my tube McIntosh C-22 preamp for a solid state McIntosh C-28 preamp. I got $140 trade-in. My wife said I could not keep both. The dealer promptly sold the C-22 to a person in Japan for $800. This was long ago. I see the C-22 is still highly regarded. The good news is, I was given a C-20 later and I still have that. I still have the C-28, also.I also blew up several power amps in the basement with a Yamaha PS190 keyboard. Lately my solid state McIntosh 2300 power amp (128 lbs.) seems to be working so far with the keyboard. The blown power amps were fixed at a reasonable price by an audio genius in St. Louis.

Linn CD-12, I bought it new in 1999 for I think it was $12,000, maybe $20,00; not sure.  I loved it, but when it died less than 5 years later. Linn said that they couldn't get the parts to repair it. I still have my 1981 LP-12, but never again will I buy a Linn product. No support on a top dollar component, 

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.