This is a weird one. I had a guitar that I borrowed from a cousin, and it was a truly awful thing with nothing nice to say about it. I used to plug that into a home made amp that actually sounded quite beautiful.
One day, I plugged it into my HiFi system in my living room, stuck in a tape, pressed record and played. It sounded like you would expect, a guitar running into the mic input of a HiFi and then recorded on tape. The magic happened on a day that I had set my guitar on the side, but still plugged in. I was tape to tape recording a mix tape that day, and must of accidentally brushed the strings on my guitar.
Later on, I was listening to the mix tape and during a song, heard my guitar playing. That began my years of fascination with recording with overdubs! I would put a tape in the HiFi, tape to tape it and play guitar over the top. I would plug in drum machines into that Mic input, record that, plug in a cheap Casio keyboard, overdub the drums, record rhythm guitars and then lead. God knows how you could hear anything on the 5th or 6th overdub, but I used to take them to school and play them in the common room!