What has been your costliest mistake in this hobby?

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.

My costliest mistake was not stopping while I was ahead and thinking by turning over gear to try I could go back and re-buy the pieces I liked the most! Wrong! Not so easily done any more.

denverfred, some of the B&K receivers were problematic and were worth walking away from. But their two-channel gear is built well with little problems if any. You would want to be careful with some of their older amps because their mosfet outputs are not available for individual replacement and you would have to opt for a complete expensive re-build on both channels.

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!

Absence of knowledge is a killer.
My biggest mistake was buying speakers that dipped below 1 ohm (9 Kappas) and pairing them with power amps that had a lot of watts, but only a 4 ohm output rating (2 Carver M-4.0t). To make matters worse, I was going to change the crossovers on the speakers and I had bought a couple powerful subs with passive crossovers to try to see if I could still get the speakers to open up, but never got the chance due to a fire.