What has been your costliest mistake in this hobby?

How exactly do you usually bias a tone arm? Eh?

Actually now that I think about it the most costly mistake I made was CD. It came out right after college when I was broke and moving around and storing my Technics and records. Bought into Perfect Sound Forever. Only years later when I dug it out of storage and hooked it up did I realize my mistake. Still had kept some records, but gave most away. Listening to stupid CD BS cost me thousands.  

Made up for it since though, let me tell you!

I've rewired my room, whole panels, component mods, all kinds of stuff going back to building a Dynaco ST200 in high school. Trying hard to think, there must have been something accidentally dropped, or scratched, or burned out somewhere along the line. Sorry, but I'm drawing a blank.  
There was a made in China Jungson amp that turned out to be a total piece of defective crap, two reps from China even came out to see first hand the noises it made, lied to my face and then refused to honor their warranty. But my dealer stepped up so even the royal mistake of buying made in China didn't cost me anything but grief and frustration. 

Honestly the one bonehead mistake I made was putting a 0.8A Orange Fuse in the Melody. The value on the factory fuse was barely legible, so that was a factor, but how in the world was I so out of it as to think a 50WPC amp can run on less than one amp? Dope. So it blew. Instantly.  

Then of course I realized what happened. Told the dealer. And surprise, they replaced it anyway. So sorry, no sad stories here. Just happy endings: Buy American. Buy Synergistic. And they lived happily ever after.