What has been your costliest mistake in this hobby?

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. 

Also blew out one channel of a nice Yamaha Receiver in the 80's.....disconnected the speaker to paint, the wires shorted and the unit was on.  Learned that lesson....

Maybe spending inordinate amounts of time on line trying to decipher what will catapult my system into nirvana. 
Champagne tastes on a beer budget is difficult. 
Democracy (read as spouse) can be a limiting factor.