What has been your costliest mistake in this hobby?

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. 

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

Very sound and precious advice for all.... That confirm my experience , when you have a relatively good audio system upgrading a part is most of the times a bad idea.... Embedding the three dimension of any system gives way more S.Q. What matters is only the embeddings mostly, especially with well choosen basic good gear ....I learn it like you the hard way.... :)

I paid for my dac a small amount of money but well choosen i never look back....

Sorry for your near 2300 dollars lost djones....