The World 1-2 Warp Zone

World 1-2 is an underground level with a maze of pipes near the end. Most players walk straight through it. A few keep exploring upward instead, and find a set of blocks forming a hidden staircase up above the visible play area — a strip of solid ground the camera never normally shows you.

Once you're standing on top of the maze, you can walk along it to a row of three pipes, unmarked and otherwise unreachable, each leading to a different later world (2, 3, or 4). That's the famous "Warp Zone."

How to trigger it: in the underground pipe maze of World 1-2, find the run of blocks that lets you climb onto the ceiling of the maze rather than passing beneath it, then walk right along the top until the warp pipes come into view.

Is this actually a bug?

Mostly no — Nintendo placed the warp pipes deliberately as a shortcut for skilled players. What's a genuine engine quirk is how you reach them: the collision system happily lets Mario stand on the topside of level geometry that was only ever designed to be seen from below, because collision in this engine is checked tile-by-tile against Mario's bounding box with no idea of "which side the level designer meant you to approach from." Reaching the warp zone exploits that blind spot, even though the reward waiting there was intentional.