World −1

This is the one that's an actual bug, not a hidden feature. Instead of walking cleanly along the top of the World 1-2 maze to reach the warp zone pipes, you can approach the last pipe from a specific angle underwater and swim into the wall beside it rather than the doorway itself.

Do it precisely enough and the game drops you into a pipe entrance it never intended to load — one that points at a level slot that doesn't officially exist. What loads next is an endless, glitched underwater level that never lets you reach a flagpole: touch a pipe at the end and you're sent straight back to its start.

How to trigger it: from the same underground World 1-2 maze as the warp zone, approach the final warp pipe from the left while swimming, hugging the wall so Mario's hitbox overlaps the solid block beside the pipe rather than the pipe's opening.

Why "−1"?

The world-select screen doesn't have art for a negative number — it reuses the same tile the HUD uses to draw a coin-counter digit. The level-select code reads a world number out of memory that was never meant to be reachable, and that stray byte happens to line up with the tile that looks like a minus sign. It's not really "World −1" so much as the game faithfully drawing whatever byte it was handed — see The Logic for why that byte was wrong in the first place.