Surely if that was the only goal, it would be easier to have a debug menu come up when you enter a certain sequence of inputs? Among other things, getting to the warp zone still requires playing 1-1 and 1-2, and you can’t warp just anywhere.

It could even only work for the P2 controller, or use inputs not normally possible like left + right simultaneously.

> use inputs not normally possible like left + right simultaneously.

Are there NES games (or mods) that have different (useful) behavior when an emulator permits such inputs?

In SMB, you can brake more quickly by holding L+R, which is impossible with an original NES controller, and as such that input is not allowed in official speed runs.

https://tasvideos.org/GameResources/NES/SuperMarioBros#Turni...

There’s controversy around this in the mario kart speed run community too. Apparently with snes controllers you can press L+R on the dpad if your dpad has been used enough to wear down the dpad’s plastic center pivot. What’s interesting is this created a schism bifurcating the community, and the one that allows L+R I hear is the more vibrant conmunity.

Not NES but I seem to recall playing Wonder Boy on a master system that had a missing D-pad and pressing up and down simultaneously made Wonder Boy drop through the earth and fall from the sky.

It's quite common for games to have bugs or unintentional behavior when pressing left+right simultaneously. Super Mario Bros. is one example (as another commenter points out); another example is Super Metroid (for the SNES), which allows you to repeatedly wall jump in-place if you hold left+right: https://wiki.supermetroid.run/L+R_Walljumping

This sparked a mild controversy recently in the speedrunning community: L+R walljumping makes it possible to climb walls underwater without Gravity Suit or Hi-Jump Boots. Through the use of very esoteric and difficult speedrun tech, this enabled a runner to complete the game while collecting only 13% of available items, which would beat the previous low% record of 14% if left+right runs were allowed on the leaderboard: https://wiki.supermetroid.run/13%25_xCharge

You just compile a dev version of the game during development.