What has been your costliest mistake in this hobby?

Buying excellent components in the first place based on the recommendations of others before I had developed the expertise to really evaluate it, then selling it for better (?) stuff to 'improve' my system.  I knew nothing (to speak of) about synergy or set up. There is quite a bit of stuff I wish I still had which is now considered quality vintage, is hard to find, and is quite expensive. Go figure.....:-)

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.