sylvie

Observation: The NES controller pinout has two pins which are usually unused (they are reserved for special hardware such as the Zapper). Therefore it would be relatively (compared to other common NES mods) straightforward to modify a NES controller to add an additional button which pulls one of the extra lines high when pressed, and to modify an NES to tie that line to short a connection within the NES case or otherwise perform some kind of action which would corrupt the state of memory, corrupt the state of the CPU, or something else that would possibly not actually crash the game but which would have a noticeable unpredictable effect on gameplay.

(I was reading about the IKAAAE Game Genie code today, which I honestly don't know what it "actually does" but apparently has an effect in almost every NES game and usually one which is "glitchy" in an interesting way. If one were willing to do more work, one could make a customized Game Genie that applies the IKAAAE code but only while the Sylvie Button is held down.)

ikno wat will ahppen. ibelieve in anything

that happens anyway you need to be able to recreate whatever it is to unlock it from behind the button.