What Disc-based games do you know of that had secrets on them when you put them in CD players and PCs?
I think probably the most famous of them is Castlevania: Symphony of the Night on PS1, where if you put the disc in a normal CD player you get a bonus music track.
One that I strongly remember from my childhood is that Sonic Adventure and Sonic Shuffle had a set of PC wallpaperson them that you could access if you put the disc in your PC.
Another interesting example was that Tiger Woods 99 PGA Tour Golf on PS1 had the South Park pilot on the disc as a dummy file.
Every single Saturn disc had something on it when placed in a PC. Maybe just a simple text file but in many cases there were developer messages or art.
I am taking about Japanese Saturn discs mind you. I have no idea about US or PAL discs.