What has been your costliest mistake in this hobby?

Buying a power amp on reputation alone. Bought an Audio Research GS150 thinking it had to be great. It was ok but hated the non responding power meters. I'd sell it in a heart beat if I could.

https://forum.audiogon.com/discussions/coda-audio-video-based-in-san-francisco-is-running-a-fraudulent-operati

 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.