First get to near the end of World 1-2, where the pipe is to get out. Jump on it and break the first two bricks above you so that one is left touching the pipe. Now, turn around and try to jump backwards through it, watch out, you need to have a portion of the pipe loaded or you'll be trapped and doomed to die. This is easier by ducking and jumping. If you succeed, you should go through the wall into the hidden warp zone. But there is one large problem, the Pirannha plants which sometimes seem to appear out of nowhere and to screw you up for a bit but then go down the first pipe. Some people say that if you are Fire Mario it will be a bit harder but after some hard work: Voila! The Minus World. But seems that if you reach the warp-zone using platforms and climb up to the top of the world that you will get these rather normal choices; left pipe takes you to World 4-1, the mid pipe takes you to World 2-1 and the right pipe to World 3-1 (Tested with jNES emulators for android and for PC).
Getting out Edit
You can't get out. When you get to the end of the level and go through the pipe, you are taken back to the start of the level. with no way out. The only way is to turn off your game or lose all your lives. (However, the Famicom version WAS beatable, and more minus levels followed) though in the NES version, you can use a level editor to add a flag. In the Famicom version, world -1 is just like world 1-3, except underwater. Worlds -2 and -3 end with the floor you see when you fight Bowser, except nobody is there. You grab the axe and the level ends with it telling you that your princess is in another castle, but yet again, nobody's there. -4 exists, but is a black screen. While in the western version World -1 is essentially World 2-2, -2 World 2-3 and so on. Similar thing happens in world 8-4 if you use the flagpole and you'll be in World 8-5 and so on where seems that World 8-6 works normal except for the colors being messed up and people joke that Mario ate too many mushrooms and he is about to puke. If you use the flagpole in for example World 1-4 you'll be in World 1-5 which is actually world 2-1 and so on. World 9 in Super Mario Bros 2 Lost Levels is more an easter egg than a glitch and you access it if you beat the game without warping (edit: it's not necessary to beat the game as Luigi as was originally claimed) and is not glitched (Tested in Super Mario All Stars in Super Mario Bros The Lost Levels although not yet confirmed for the NES version).
Other Glitch WorldsEdit
Super Mario Bros. actually has 256 worlds total, but all of the worlds past 8 are usually glitchy and broken. The Minus World is the only "Glitch world" that be can accessed through the western NES, while many more can be accessed though a Japanese Famicon. In Super Mario Bros.: The Lost Levels, there is World 9 which is actually more an easter egg and you enter it if you beat the game without warping.
Credits and acknowledgements Edit
http://www.youtube.com/watch?v=COfIWs2PnPM - Video of how to get there.
Happy glitchin' :)[
- World 1~2-x is easy (except boss world).