The infamous Minus World is a glitch in the original Super Mario Bros. for the NES that when exploited, allowed players to travel to a water world labelled as -1. The level design has the same layout as world 2-2 and world 7-2.
The downfall of this world, is that there is no way out. The pipe at the end of the stage just takes you back to the beginning, and does not reset your time limit. The world is basically just an endless loop of doom.
So why would you come here? The only reason I can think of is the experience. This is an area of the game that was never meant to be here. In Mario's universe it's like an uncharted dimension with no return.
If you would like to explore the minus world but aren't quite sure on how to get there, this page is here to help you out. It's a little tricky to accomplish at first, but you'll eventually nail that lucky jump that takes you there.
Make your way to the end pipe in world 1-2, but make sure you do not enter it.
Smash out the missing blocks you see here, and stand on the very edge of the pipe.
Jump backwards from the edge of the pipe against the remaining brick block.
Eventually you will hit it just right, and slide through the bricks, the pipe, and the wall.
When you get to the other side, hop down a pipe before the Warp Zone text appears.
You will now find yourself in the endless loop famously known as "The Minus World".
I'm not sure if this glitch is that well known or not, but I know it used to bother the heck out of me when I was a kid because I'd get stuck by accident and be forced to let time run out. When going for the beanstalk that takes you to the warp zone (worlds 6-8) in world 4-2, it can disappear from the screen if you walk too far to the right. It's still there, but now it's invisible and touching it will hold you in place until you die.
I have attempted to glitch out other beanstalks in other worlds, but oddly enough this is the only evil beatstalk that fades into the shadows and entangles you forever. No matter what you do, you cannot break free. You can jump around all you want, but time will eventually run out.
This section will show you how to glitch out the beanstalk and get stuck if you want to try it for whatever reason. It's actually quite simple, but it takes a few nudges to get it exactly right. If you are playing on an emulator, make sure to create a save state so you can reload.
Make your way to world 4-2, and open up the hidden block that holds the beanstalk.
Slowly edge to the right of the screen, until the beanstalk seems to disappear.
Go ahead, run up and touch it. You will now be stuck, so I hope made a save state!