What has been your costliest mistake in this hobby?


For example :I recently learned a hard lesson- I accidentally ran voltage thru my $3000 MC cartridge (kiseki purple heart).  I have a TT with 5 prong connector and a phono cable with a 5 prong connector.  I accidentally swapped where they plugged into and ran electric thru the tonearm into the cartridge.  It was a stupid - not thinking- hasty mistake. When I corrected the problem the cartridge was fried.  An avalanche of four letter words followed!

So what has been your biggest and/or costliest mistake?
polkalover
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. 
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.