What has been your costliest mistake in this hobby?

phd:
Right you are. In 2001, I really liked the sound of the B&K electronics. It took less than 10 sec. to pick it originally vs. more mass market brands. Hot sucker! I almost bought one of the 2ch amps when I kicked HT out of bed, but why tempt fate? I used to have a dickens of a time selling against B&K. Who'da thunk they'd come and bite me in the butt after they were dead?
BEWARE! VAMPIRE AMPS!

Yeah, I went thru the dimpled dome experience after a flood. The complex’s lawn irrigation system sprung a huge leak next door and left my basement listening room with about 3" of water. Emergency crew moved my Dynaudio C60 speakers. Yes, I should have done it myself, but in the panic . . . .
My most expensive mistake was buying a B&K AVR307 from a local dealer--a former employer. Great sounding receiver in all respects, but when rear-channel noise showed up I took it in for service. I was informed that they no longer carry B&K, that B&K was out of business with no parts access. I was so disgusted I didn’t even protest. I knew if I did, I would eventually have to insult my old boss, who I still like, and the service manager, with whom I always had a good relationship. I walked out, leaving the B&K sitting on the counter. $3000.
I later followed my audiophile instincts, having discovered the limitations of HT for my idea of Hifi.
Now that I have surrendered myself to fuller immersion in this hobby--a solitary exercise in constant frustration as the transitory sounds of whatever comes out of the speakers gets evaluated against some ideal swimming around in my memory--I realize that I should have put the thing up for sale for parts.
I’m still pissed. But I should have known more about B&K at the time. Online sources weren't what they are today and I trusted my old employer too much--especially a service dept. that had never lived up to standard when my customers had problems years before. Live and learn.