What has been your costliest mistake in this hobby?

Going through loads of mid-fi equipment in the vain hope to find, by sheer ’magic’, some satisfying sound quality. Didn’t.

’Außer Spesen, nichts gewesen’...

Maybe the learning exercise was of *some* value.
The sound quality was just NOT getting there until very many years later.

Good sound doesn’t come easy - and not quite at bargain prices either. 😕
Be surprised if YMMV.
M. 🇿🇦
PS: oh dear, and one more nasty learning exercise.
When I connected my ML39 CDP, used as a pre-amp, to the digital satellite TV decoder.
One lightning strike, - the connected coax cable screen/ground must have gone sky-high, some 10k Volt for a split second and fried the Levison no.39.
One stinker of a repair bill. 

Some years later, I had a connection, to my ML36 DAC, from my DVD player, to my TV, to my Decoder -
and from there, wiress to the good Lords lighting.
Boom, - decoder, TV, not the DVD?!?, but the DAC nailed.
So, so sad, eish! 😭

Lucky enough, not too costly as both items pre-owned.
— Transparent Digital Reference and
— Transparent XLR Ultra Music Link. 

Both cables don't live up to promise, despite their rather elaborate constructions.
SAD. 
M. 🇿🇦