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
Lol great idea for a thread!

For me, impulsively buying speakers before I realized that I shouldn't be "collecting" them.  
Audioquest cables (great product.......but)

OR

Now using Mogami, Canare, or Gotham from worlds best cables
for interconnects and speakers.
14 gauge power cords into a dedicated breaker and hospital grade outlets.
Power conditioners

......same cost
Found this out the hard way.
The costliest part of this audio journey for me is my inability to sell gear.
I sure I'm not the only one afflicted with this disease. The upside is that I find Easter Eggs when I'm hunting for something in the house that is unrelated.  Recently found three Hafler amps in a closet and a GAS amp (remember those?) in the attic. 
The only piece I ever sold was a Teac A-3340S 4 channel RTR, because I had two of them!
 
Upon leaving college I "sold" a pair of B&O speakers to my room mate. They were very good at the time mid-70s.........costly yes, because I never got paid for them.