The silencer amp
In the early 1990s I was playing with MOSFETs in an attempt to build an extremely loud audio amplifier on the cheap. I built 4 Class AB amps on a +/- 45V power supply that could manage 8A momentarily. The circuit design was crude and involved twisted hookup wires between the MOSFETs and the main breadboard. The final build suffered from a bit of distortion and my old oscilloscope showed bits of the waveform missing, which was a typical symptom of intermittent oscillation. I soldered some picofarad caps on the MOSFETs as an easy fix. Now it was weird. Every time I turned on the amp, the MOSFETs hissed, my radio went dead, my CD player spun BACKWARDS, and my oscilloscope went totally blank. I disconnected the oscilloscope probes and it still wouldn't work just being there near the amp. The amplifier felt oddly hot where it shouldn't.
Moved the oscilloscope and powered on the amp. The scope a drew what looked like spring viewed from an angle, which means that the trace was partially going backwards too. I measured the loops and got 170 MHz on a 'scope rated for 20MHz while its probe was not connected. It turns out that the pins on a TO-220 MOSFET make a fine RF transformer. You're supposed to put series resistors on the pins to stop oscillations; never capacitors. I was energizing all the speaker wires in my room with 170 MHz of high voltage AC. The amp felt strangely warm because it was RF heating my fingers when I touched it.
I eventually got it working and it was worth every bit of effort.