What has been your costliest mistake in this hobby?

Got a used Furman it-ref 15i. It made the sound dull. Yuck  Plugged AMP right into wall and life was good. Wall outlet upgraded too. 
Then got a new niagara 7000 and all was well. 
Put the Furman on the “B” system and it didn’t affect sound.
Stuff fits together or it doesn’t.  
Wasted $ on poor connects and speaker wire. Didn’t know enough back then. 

That's an easy one...

The tweak in which I was directed to take a Polaroid of myself bathing in a bathtub full of Diane St. Clair butter and then afterwards to place the photo in the kitchen freezer. 

The butter alone cost over $18,000.00. 

I mean what was I thinking (should have used the smaller tub in the spare bathroom)? 

DeKay 

Telling my 3rd wife what I spent on speakers.

I bartered with a very well regarded local dealer.  Made the mistake of thinking we were friends.  Traded a very large, very expensive oriental rug without agreeing on what I was to receive in exchange - we agreed that he would have rug valued.  Took a long time - months? - but eventually we agreed on a value, $10k.  With that, he was supposed to source a Garrard 401, build a plinth, and also I was going to get a new DAC.  He never delivered on anything - I would nicely remind him, and he’d make some encouraging noises, but 3 years later I sent an annoyed note and he freaked out. Said I was being unreasonable, we didn’t have a deal, blah, blah, blah.  Eventually, he had a guy from his shop dump the rug on my driveway when I wasn’t home. Had a new stain on it too.  Obviously haven’t talked to him since.  Oh well.  Found my own 401 in U.K., shipped it directly to Woodsong Audio (Chris Harban), who restored it and created a gorgeous plinth for me, color matched to my italian tube amp (purchased from the a-hole).  Still haven’t replaced my DAC or sold the rug, but got a beautiful tt!

One I just made buying an Aurender N100H which doesn't sound any better than the raspberry pi4 I using with the same DAC. Don't fall for the Streamer matters just as much as the DAC, BS.