Super Mario Bros./World -1

World -1
The World -1, also known as The Minus World is a hidden world which is the result of a glitch in the Warp Zone in World 1-2. The reason why it's called Minus World it's actually because it's not essentially a whole World. It's basically World blank, which only consists of only one standard underwater-never-ending level.

If you wish to access this glitched World, you need to go to World 1-2, at the very end of the underground part of the level, shown on the image above. Do not enter the pipe that leads back to the surface. Instead, stand on the pipe and break the second and third ceiling blocks next to the pipe. It is important to leave the block directly next to the pipe intact. Then stand on the far left edge of the pipe and continue facing left. Here's the tricky part: you must make Mario jump up while crouching and to the right while still facing left. If you hit the remaining block at the proper angle, Mario will pass through it and begin walking through the brick wall. Soon you'll enter the Warp Zone room. Drop into the first pipe on the far left, and you'll be whisked away to the Minus World.

Explanation
The main reason why this glitch happens is because of the camera glitch. When Mario goes through the wall, what he's doing is going before the camera does. Thus, when Mario access the Warp Zone before the camera, those Warp Pipes do not lead where they were intended in the first place. Actually, the first and the last pipe lead to the World -1 while the pipe in the middle will actually lead you to World 5.