What has been your costliest mistake in this hobby?

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.

Getting married.

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.   

Having UPS deliver to me or from me. Never going to happen again.

Biggest mistake I made was buying a pair of highly regarded floorstanding speakers which got many great reviews without ever hearing them first.  They sounded God-awful so naturally it couldn’t be the speakers.  I replaced every component and wire in the system at a tremendous cost and after many tens of thousands of dollars, the system still sounded horrible.   After many sleepless nights, I sold the highly regarded speakers and started over.   Lesson learned.  I will never buy something like speakers without hearing them first.  That mistake cost me upwards of 40K.