What has been your costliest mistake in this hobby?

Mcintosh  MVP 891, thinking it would be good($5000)

     Shorted out all of my Audire electronics!     Two amps and two preamps (used to keep subs in phase while being able to separately control bass), and my CD player were fried.  The battery powered head amp had been my mainstay. 
     When an old college roommate reversed the batteries in this POCO, it killed it, just as it had in college when the same idiot did this. I was without, so I traded it and one of my Diffet 3's for a Diffet 2 with the POCO built in. 
     About a year later, the Diffet 2 started taking longer and longer to come on.  I posted a photo of its innards, and a nice person on the internet pointed out two little caps that powered up a pair of reed switches. If I had left it on, there would have been no issues, at least for a while 
     I ordered the caps and arranged to get them soldered in, but first, I had to get involved and decided to test the transformer.  Putting the test leads on the terminals, to check output power, I shorted a lead against the silver wire that provided power to a row of 120 volt outlets, thereby sending 120 volts through the entire system output interconnects, and apparently through the CD inputs, somehow missing the KT-917 tuner and XM radio, which must have been switched out. 
     I was at least grateful the tuner was not damaged.

I've been pretty lucky with my purchases, even luckier connecting with people who gave me great advice, which guided me to "listen for myself".

...Not really a mistake I made with purchasing equipment (or damaging a component,) but about 15 years ago, at the launch of the CD format, I sold a perfectly-working Lenco L78 TT for about $75! Today, I see on various Vintage equipment forums that they fetch some $550...Silly me...

My biggest mistake?...I'll call it experience that led me to spending money.
Went to a local home / dealer to audition a Parasound Halo Integrated amp. It was right in my 'wheelhouse'.for price point and has excellent reviews. It sounded good.  We listened for a bit then he swapped in an Esoteric DAC / and I don't know what else and I was blown away.  I stated it sounds like a 'completely different pair of speakers'.  First I really learned about the quality and impact of the electronics..not just the speakers.  So there I went, not Esoteric, but down the road of several upgrades and trade ins  and now have arrived at music that mesmerizes  me and is my escape.
 
toug27: "I decide to install a secondary system in my basement to let my wife sleep better while I could listen to live music level downstairs thinking that I could be happy with a basic system...read limited budget...God how naive"

This was also for a basement 'second system'  It is now better than my main system upstairs!

Buying and selling some equipment did cost some, but it was the price of learning.