Years ago I bought a used original Game Boy so I could scavenge the screen off of it, to replace mine which was failing. When I tore it apart, all the buttons had a yellowish brown grime that resembled ear wax on them. It was all inside the case, so I'm not sure what it was and it looked pristine on the outside. I gagged so much when cleaning the buttons, etc. I got it for $10 with a game, so it was a cheap but gross repair.