When I was in HS, my teachers became wise to the existence of apps that would display the mem cleared message, so they would go around to each student and clear the memory themselves. This annoyed me as I'd written a lot of useful to me utilities (not cheats) and games, so I wrote a not-so-little program that emulated all of the system menus. You have to enter a hidden key combo to get out of it.
Looking back at it, the code embarrasses me because it is so goddamn awful. I didn't know how to write functions or use the stack properly. It was just one giant mess of jumps and conditional branches. But hey, it worked.
Sadly, I think all of my Z80 assembly is lost. I don't think I shared it on any sites, either.