What has been your costliest mistake in this hobby?

STEALTH cables sold them all they have no resale value in the USA STAY AWAY overpriced 0 resale.

Paying too high of a price for a pair of Quad 57's and then, years later, having to sell them at a loss! The smart thing would have been to just keep them! But I needed some fast cash!

I'm going to exclude buying stuff that I ended up not liking, because purchases of audio gear are always experiments.  So the stupidest thing I did was trying to disassemble a ribbon tweeter trying to see if I could upgrade the wiring to and from the tweeter.  Destroyed the ribbon in the process of disassembling. 

I thought it would be fun to hook up a keyboard to my system and play some music. Due to a handling accident, I accidentally sent a 12V signal to my amplifier which proceeded to amplify that to what I calculate was over 140 volts with a fair amount of DC included. Needless to say, the speakers didn’t like it and refused to play again. The silver lining is that I replaced them with a pair of DIY speakers that I never enjoyed, but then learned of a design error I made and now they are my favorite speakers I’ve ever owned. Much better than the pair I blew. I never would have figured it out had I not blown the other pair. 

Reading reviews like if all pro-reviewers know what they talk about....This take me to bad choices...

After that i read simultaneously all non- pro- reviews and not only the pro-one about a product and succeed to buy my actual audio system and i dont look back anymore...Studying non professional reviewers and professional reviewers about a product is mandatory...

The costlier mistakes in this hobby is the universal consensus around the word "upgrade" and the sheeping crowd march toward frustration and the money pit, except for those who can afford the "illusion" given by an "upgrade" to the top of the line product...This is a myth, this is nor true nor false, this is an half truth....Worst than a lie....

The truth is ANY good audio system, not necessarily costlier, to attain his peak, to reach his optimal workings, must be rightfully embedded simultaneously in the mechanical resonant dimension, in the electrical grid and in the acoustical space....

The higher price of electronic components are testimony of their design quality yes, but created the illusion that all electronic design component is perfect to use ready out of the box and will give the better of itself without any other work by us.... This is false...This is a mandatory marketing lie....Nobody can sell a 10,000 amplifier saying that after buying it the job begins and some other works are needed from us.... 

Who will buy a problem instead of a solution?   :)

The price does not give you the audiophile experiences by itself, the rightful triple embeddings controls work will, at almost any relatively low price if you are creative....