What are some consoles that had hidden things you could do? I know some consoles had expansion slots, or other slots on the bottom or side of the console. I always wondered what the NES's bottom door was for. I imagine you could install something there at some point. But what are some hidden things consoles can do? Maybe there's a secret menu you can get to, a connection like I mentioned above that takes a certain cart or adapter. I know the Sega Saturn for example had an expansion slot, but I never knew what it was for.
I know there is a slot on the bottom of the N64, that could connect to a 64DD device, to I think add extra ram or something, or maybe a CD drive. I forget. Anyway, it never came to the states, but it was in Japan I believe. Actually, wasn't it for online support?