What has been your costliest mistake in this hobby?

Cable drop, I added a whole new, meaning to four letter words..
I'm still fixing stuff. A single 300 deluxe VTL, a GG MC275, and two months after that I lost a channel (same side as the cable drop) on my C2500. 

Cost to replace off the shelf, 12K plus. Cost to repair, A LOT of time and money over 2,000.00 in parts alone. TWO transformers, not a single blown fuse, only at the main, THE MAIN. It popped a 20 amp breaker, but not a 5 amp fuse, on the surge protector, amps or preamp...

I must have looked like the Abominable Snowman, after he got his teeth pulled... Yup that was me.

Now I call PG&E and have them shut down WHOLE grid, when I work on my stuff.

Yup, you can blame me, for no power this summer, not the fires.
Doing a cable swap, "Shut down the grid" oldhvymec is at it again. ;-)

Regards

I forgot this one, but it was almost made right.

I went out into my shop one day and it was cleaned out. This was 6 years ago. My C22, 2 MC240, 4 TTs, 2 Reel to Reels, A foot locker of NOS valves (est value 25,000.)
A handbuilt, 11 band, tube, parametric, EQ with 2 external 3 way crossovers. My stratherarn ribbon speakers.

ALL the gear was packed in foul weather shipping crates.

A lot of tool and other things too.

The insurance paid 56k or something like that, The only big loss was the strathearn ribbon speakers, I couldn't find a source at the time. so they wouldn't replace the 6K or so for them, only depreciation of 2K or something..

BTW know who did it, told them they did it. A kid I helped out. It didn't go well for him. His bad ways caught up with him, sad...

I did ok though, considering.. First and only time someones done that to me..Hurt my feelings... Yup sure did.. Helped raise that kid...Little turd...

Regards