What has been your costliest mistake in this hobby?

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. 

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.