What has been your costliest mistake in this hobby?
For example :I recently learned a hard lesson- I accidentally ran voltage thru my $3000 MC cartridge (kiseki purple heart). I have a TT with 5 prong connector and a phono cable with a 5 prong connector. I accidentally swapped where they plugged into and ran electric thru the tonearm into the cartridge. It was a stupid - not thinking- hasty mistake. When I corrected the problem the cartridge was fried. An avalanche of four letter words followed!
So what has been your biggest and/or costliest mistake?