What has been your costliest mistake in this hobby?

My costliest mistake was going down the VPI rabbit hole. Based on reviews and specs, the VPI Classic (with the SoundSmith cartridge specifically designed for it) was my dream analog rig but it became a money pit and a source of endless frustration.  After a year of back-and-forth with VPI, SoundSmith, two local shops, and a setup specialist all attempting to dial out problems, I just gave up. Even at its best, the once-simple act of putting on a record and blissing out became convoluted to the point that I simply listened to my records less often. In the end, I was able to sell it for close-enough to what I paid and replace it with a beautiful sounding—and looking—new table (PearAudio Blue) that was simpler, smaller, less prone to falling out of adjustment, less prone to skipping, and generally a joy to use. The simple pleasure of putting on a record and losing myself in it returned instantly. An added bonus was that my family, who were afraid to touch the VPI, have no problem with the new 'table and have been listening to more records as well. 

Bought a Krell HTS 7.1 for $8000 MSRP. Dropped in value like an anvil out the window. Sold for only $800. OUCH! Technology sensitive products drop way faster than the market for High End stuff.

1) not listening before buying
2) not properly packing before shipping
3) not figuring out synergy between amplifiers/speakers
4) not considering room’s impact on sound

I'm sorry, you said just one.
  

Ham fisted and connecting arm cable to a high end Dynavector and pushed the connecting pin into the cartridge.

Bought a pair of Wilson WATT3/Puppy2 new in the mid-nineties that I thought was the last word, but eventually took an 80% loss at resale for a better speaker at less than half the price.  Ouch!