Super Mario Bros. (NES)
World -1 (also known as the "Minus World") is a looping water level, only accessible through World 1-2. Here's how to get there: As Super Mario, go all the way to the green pipe at the end of 1-2. Stand on top of the pipe and smash the blocks above you except for the very last block on the right. Next, stand on the end of the pipe and face left. Jump straight up, then press and hold Right on the directional pad. You'll notice that Mario kind of "slides" through the bottom left corner of the block backwards. (You can also try ducking before jumping so that you don't risk breaking the block.) If you jump at the right angle, Mario will be pulled through the wall. Wait for Mario to pop out of the other side of the wall, then go down the first pipe before the "Welcome to Warp Zone" message appears. Instead of warping to 4-1, you will go to -1 (technically, it's "World [blank]-1"). World -1 is just like 2-2, but the pipe at the end takes you back to the beginning. You cannot beat -1, but it does give you lots of swimming practice. As for the other pipes in the Warp Zone, if you enter them before the welcome message appears, the second one takes you to 5-1, and the third one also takes you to the Minus World.
In the Famicom Disk System version of SMB, the Minus World is a swimming version of World 1-3, with gray enemies, pink platforms, and a floating Bowser. At the end, if you touch the flagpole too high, you'll float over the castle door and won't clear the level. Touch it about halfway down to be safe, and you'll drop right into the door, taking you to World -2! World -2 is exactly like World 7-3, except beating the level takes you to -3. World -3 is like 4-4, except it's not a maze, there's no Bowser at the end, and the graphics are blue instead of gray. After you touch the axe at the end, you are thanked as usual, but the Mushroom Retainer is not there. Press B, and you are taken to the title screen, where you can keep pressing B to choose a World, exactly as if you had just beaten the game.
In other versions...
Fixed in SMAS? Yes.
Fixed in SMBDX? Yes.