The dumbest thing you ever did to your stereo?

Broke a Koetsu. Beers and vinyl don't go together ...

Played frisby with woofer having a rotted surround. One day before learning that speakers could be reconed and that manufacturer had sold last original equipment replacement woofer one month earlier. Ouch, ouch, ouch.

I attempted to drive a set of Infinity RSIIs with a Yamaha M2.
please note the RSIIs could and would drop down to 1/4 ohm and the M2 is only rated for 8 ohms. It was like having Mohomad Alie and John Frazer in the same room. The amp was trying to destroy the speakers and the speakers were trying to destroy the amp.needless to say I was very upset.

Tube rolled 4 times with NOS McIntosh originals, Telefunken, Amperex, GEs and Mullards trying to tame my C2200 McIntosh Preamp of its shrill upper end. Constantly bought NOS and quickly resold at a loss on A'Gon because everything sounded harsh despite claimed warmth of those tubes and preamp. Was just about to send it in for warranty work when one day while sitting trying to listen I remembered that the C2200 had a treble dial - which I'd forgotten about since none of my equipment has had it in the past 15 years. So, I walk over to the Mac to turn down the treble from neutral and discovered the dial turned all the way up.
Then remembered my 2-year-old grandson had been left in the listening room unsupervised for a while just before it happened. Musta had a lot of fun playing with the cool dials. Turned the treble dial to neutral, everything sounded great. But audioplila nervosa set in again and the tube-rolling bug hit again. But now I am happy. Poorer, Chagrined. But happier. My son will never let me forget it.

Do headphones count? I managed to ruin a pair of vintage Stax Lambda Pro Signature earspeakers by trying to suck out the last few crumbs of degenerated lining foam with a vacuum hose. Not a good thing to do for mylar diaphragms. No siree.