The dumbest thing you ever did to your stereo?

I was just curious about other peoples horrow stories. A few months ago, I disconnected my preamp and inserted another preamp. When I turned the pre on there was a horrific screeching, high-decibel, other worldly noise eminating from my new B&W diamonds...honestly, it sounded like an off-tune Scottish bagpipe army on steroids. The house shook. My heart was pounding and I quickly ran to the wall outlet and pulled the plugs. I just stood there in shock...just like when you just miss getting into a car accident. I was sure I damaged something...but alas, everything is alright. Because I dismantled the preamp so fast I still don't know what the hell I did. I'm thinking I may have mistakenly misplaced the interconnects or something. Stupid but lucky.