What has been your costliest mistake in this hobby?

Other mistake was selling off some non-audiophile vinyl thinking cds would sound better. Didn't get a cd player until it became apparent that new records were not being released on vinyl. I am still trying to catch up and in many cases only able to find non-audiophile vinyl to replace the ones I sold 30+ years ago. Now paying  5 times or more what I sold them for.

Can we take the personal attacks out of this please? Settle up or agree not to sell each other anything anymore.

When I was first starting out in the audio addiction when I was in college and turned equipment over sometimes on a weekly basis with the idea that it would sound better (and sometimes it did), I got taken advantage by a very unscrupulous dealer who always threw in something to the deal which would have been better if left out. The first thing that comes to mind is an Audiosource Equializer/Spectral analyzer. I saw something like it in a movie with all the lights dancing and thought a high end system had to have it.

I couldn't figure out why my system sounded worse until another dealer asked why I bought that and he said get rid of it, and of course when I did, the system sounded better than before that last swap. I learned that once you get an amp/electronics that works, stick with it. I think I lost 75% of my investment when I gave it back to the unscrupulous guy and traded it for store credit to buy records. Never bought another piece of equipment from him.

Speaking of records, I learned not to buy records just because they sound good. Buy the ones you like that are of the best pressing that is available, priced within reason. I really like Nautilus recordings (better than MFSL and most Japanese) and in my excitement over them, I got a few that I rarely, if ever, listen to. Quincy Jones  "The Dude" and Doobie Brothers Minute by Minute come to mind. Basically, most of them that I would not have owned otherwise should not have been purchased. Don't be one of those who buy records like a stamp or coins - listen to them and if you don't like them, sell them and let someone else enjoy them. Or if they do stink, at least you'll have more space in the cabinet. I have to take one out for every new one I get, and it hasn't been a problem yet.  

Nothing wrong with a legitimate upgrade if you set $$ aside to spend some "pin money", or for a forced replacement from a broken component that is not worth the cost of repair. A merry go round of change for change sake looking for the holy grail doesn't make sense. If you can actually hear the difference and are willing to pay for it and it doesn't impact your finances, why not? The addictive urge to continuously improve is unfortunately part of many areas of consumerism in our society. As you get older (and hopefully wiser) you don't make purchases impulsively (like I used to do in college, not knowing what the hell I was doing).

But if you are like me and your cartridge stylus completely wears out after 9 years (yesterday) that puts you in a state of panic because you have no legitimate back up, and the time to repair is 4 months or more after shipping overseas with the virus delaying everything, you can make a pragmatic decision.

Luckily, the manufacturer apologized for the extended wait time and accommodated me with a brand new replacement for a few hundred bucks more than a service overhaul. You thank them and will be loyal forever. I could have paid more for an upgrade, (with a similar credit for another $1500) but I was totally happy with the sound out of a cartridge that is still at least on the level of the rest of my system (probably better), and I'm going to be in the same situation sooner this time as am probably listening to my system 3 times as much as normal.

I had a similar situation with a cable that was too short after some box reconfiguration and the manufacturer allowed 100% towards an upgrade of the current price. I couldn't beleive it. It was 25 years old! I will never buy another cable that doesn't say Kimber on it. Plus, now they get free advertising on Audiogon.