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!

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. 

rickytickytwo, did you ever find a power amp that could drive the Kappas? I would think that some of the Class D amps might qualify.

Buying speakers that I was certain must be fabulous based on a review and talking to the builder...without hearing them personally first...put 'em in my system, was sure they were breaking-in, waited, waited some more, and then realized they just didn't work for me...by the time the builder had sent me the proper serial number badges (6 months at least) he wouldn't allow a return...lost a couple hundred bucks when finally sold. Good news was the speakers I already had sounded great, and I was simply looking for more efficiency. Bad news for the builder as I'll now have nothing to do with him or his products. 

Throwing good money after bad:  In 2006 I purchased a one-off Odyssey Audio HT3 from Klaus.  In warranty, it needed a new cap.  Parts and labor were free, but shipping this 64 lb beast wasn't.  12 years later, catastrophic cap failure.  Klaus was surprised it took so long to fail (!).  I shipped it to him at great expense, and I agreed to a full Kismet Internals upgrade of the L/R channels for $900 + shipping.  After a long, long wait, I received an amp apparently damaged in shipping that failed.  Klaus offered to repair it yet again, for free, but again, round trip shipping was well into the hundreds of dollars, as Klaus insisted that he return it to me via air freight (fortunately I live near a major airport).   I had it for 5 months before it failed again.  I had decided after the last repair not to lug this thing to and from shippers if it ever failed again, so it is now a boat anchor underneath a better sounding Class D amp from Arion Audio.  Trouble free for over a year, it weighs just 20lbs., so I can handle it if it ever does need a repair.  And it is one of the cleanest amps I have  ever heard.