Ok, I'll share some of mine if ya'll will share yours.
When I first got my beast I plugged it into my guitar amplifier and heard... nothing. I cranked up the volume to what would be ear damaging levels and faintly heard a varying tone as I moved near the pitch antenna. I checked the cable and the theremin's output connector with no joy. I put it on the shelf awaiting repair, and promptly forgot about it.
Many moons later I finally brought it to the bench. When I powered it up, I was rewarded with strong signals on my scope. This made no sense at all to me. I tried it with my amplifier and again had no output.
I said a very bad word.
To make a long story slightly shorter, but no less embarrassing, my theremin was sitting directly on top of my amp when it chose to be silent.
A metal chassis just inches from its volume plate.
Oops......