Back in the 90s I still had a Commodore 64 with a MIDI controller that plugged into the joystick port. I made a homebrew cartridge with an analog sampler chip that plugged into the Commodore's expansion slot. All the parts came from Radio Shack, including the chip. I wrote a program that allowed me to record samples and control playback from a keyboard plugged into the MIDI controller. Eventually I intended to add options to save and load MIDI sequences.
Unfortunately, I was a little too cavalier while tinkering with the cartridge. After making a few tweaks to the circuitry in an attempt to reduce noise, I powered on the Commodore and immediately fried the motherboard.