What has been your costliest mistake in this hobby?

Buying speakers that I was certain must be fabulous based on a review and talking to the builder...without hearing them personally first...put 'em in my system, was sure they were breaking-in, waited, waited some more, and then realized they just didn't work for me...by the time the builder had sent me the proper serial number badges (6 months at least) he wouldn't allow a return...lost a couple hundred bucks when finally sold. Good news was the speakers I already had sounded great, and I was simply looking for more efficiency. Bad news for the builder as I'll now have nothing to do with him or his products. 

Throwing good money after bad:  In 2006 I purchased a one-off Odyssey Audio HT3 from Klaus.  In warranty, it needed a new cap.  Parts and labor were free, but shipping this 64 lb beast wasn't.  12 years later, catastrophic cap failure.  Klaus was surprised it took so long to fail (!).  I shipped it to him at great expense, and I agreed to a full Kismet Internals upgrade of the L/R channels for $900 + shipping.  After a long, long wait, I received an amp apparently damaged in shipping that failed.  Klaus offered to repair it yet again, for free, but again, round trip shipping was well into the hundreds of dollars, as Klaus insisted that he return it to me via air freight (fortunately I live near a major airport).   I had it for 5 months before it failed again.  I had decided after the last repair not to lug this thing to and from shippers if it ever failed again, so it is now a boat anchor underneath a better sounding Class D amp from Arion Audio.  Trouble free for over a year, it weighs just 20lbs., so I can handle it if it ever does need a repair.  And it is one of the cleanest amps I have  ever heard.  

I’ll be a contrarian here ... the most COST EFFECTIVE thing I’ve done is to have gotten off of the upgrade roller coaster years ago. I now tend to keep equipment for a very long time and have stopped looking for the latest and greatest. It’s worked well for me, and saved me thousands.

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.