Back in 97/98 I remember having discovered hidden menus in some PlayStation 1 demo discs.
Since then I haven't read anything about that (either in specialised magazines or online).
Recently I dug out some demo discs and this little discovery came back to my memory.
Unfortunately I can't remember the trick I used to access the hidden menus, but with the help of an Action Replay/GameShark, I've been able to make them appear.
For instance in one of my demo discs there's a hidden "Tech" menu:
Entering this menu gives you access to an other "Ultra Cheats" menu:
Please leave a comment if you have more info about those menus.