What has been your costliest mistake in this hobby?

I have, on occasion, sent prime examples (VERY low hours) of tubes from my NOS, backup stash, to AudiogoN members who’ve asked about, "the best" of the 6SN7 and/or 6DJ8/6922 types, to taste for themselves.       A little over 2 1/2 months ago; I sent a pair of Amperex Navy 6922 (early Sixties/large halo getter/maybe 10 hrs) and a pair of early Forties Ken-Rads (bottom gettered/staggered plates/copper rods/less than 10 hrs) for a member to try in their Cary SLI-80 F1, based on what they described as presentation preferences.       They loved the presentation so much; they asked if they were for sale and- did I have anything else, that they could audition.        Glad I didn’t, because: it’s been at least a month of attempted communication, without response.       When purchased (some 20 yrs ago), I paid around half of what they’re worth now (around $500, today, combined).       Glad I’m not as bankrupt (financially or morally), but- that still stings.       No one to blame, but- myself (of course)!

Really......."I paid for it alright".  That's your answer to having no morals, no values, no ethics. no character, no manhood.  OK I'm sure you think in simple, one thought sentences.  I'm sure you have never asked yourself what it means to be a "man". You sold your soul for $250,  People have sold theirs for less. I'm absolutely convinced that you have never asked yourself, how can I leave this world a little better than I found it.  A lot of us work a lifetime doing just that.  We sacrifice our families, our friends, our health, and untimely die younger than the average stats say we should.
Some people would never put themselves on the front line of dangers and you are obviously one of them. Because you need character to do that. Marines have a saying...most people stand behind the flag (I'm not so sure anymore unfortunately, my addition) Marines stand in front of it.I'm sure you would never make the grade, wouldn't even be close.
You know I'm not too hard to get along with, even though I am and most people I work with are Type A ++ personalities.  You want to see some nutso geniuses and or pretty smart people or at least that's what they think they are, walk around with me a day.  But I do know the Bobfather, I even examined  him once., what an honor that was)  All I am going to do is send on exact copies of what you and I posted here, nothing else and ask him should your Silver's ever need repair would he be willing to it.  This is a man with no affiliation with some companies he was no longer with and repaired out of warranty and no charge because the products still "had his name on them"  Again, Karma has a way of coming around.  I like birds a lot, I think my favorite is the Swallow.

Never any costly mistakes, just "happy little accidents."
(Bob Ross) RIP

My costliest mistake was not stopping the purchases before I spent all my disposable income from savings. And, secondly not finding someone locally (maybe a knowledgeable dealer) who new audio (since I was a newbie back when) to help me with system-building instead of the trial and error I went through on my own buying here at the Gon. Couldn’t see that the money was dwindling slowly. Didn’t realize what I had done until it was too late. Burn and churn was what I did. Now the disposable income is spent. I can only afford small purchases - for a couple $hundred here and there. My days of spending several $grand here and there are all over. I have to look to retirement with significantly lowered savings. I spent above my means without understanding it. You newbies in the pursuit of high quality sound reproduction should learn from my experience.

Last year I moved house.  For months after the move I couldn't understand why my sound quality was apparently worse than before.  I blamed the acoustics of my new room.  

Only when doing some REW testing did it come to light that both my Seas tweeters were pressed in.  $900 a pair, plus installation.  I have no idea how this happened.  Could have been a visiting child or dog - but most likely it was clumsy carrying by me.

That being said, the new tweeters are great and my Kudos Super 20's sound better than ever.