Super Mario Bros. Glitches

Minus World
Minus World (level -1) is an endless water level that looks just like Level 2-2, except reaching the end brings you back to the beginning. To enter Minus World, go to the pipe at the end of level 1-2 as Super or Firey Mario. Break all but the last block above the pipe. Stand at the end of the pipe, jump up, then hold right trying to hit the bottom corner of the block. If you’re lucky, you will slide through the wall and into the Warp Zone before the text appears. Go down the first or last pipes to reach Minus World, or the middle to go to level 5-1.

Small Firey Mario
You Have to be Super Mario for this. Go to any of the Bowser levels. Jump over Bowser. Land on the ledge, but don’t land on the Axe. Wait for him to come to you. Then position your self so when you jump you will hit the axe and Bowser. If you did it right you will remain flashing. Now when you get a Mushroom you will shrink instead of grow and you can still use fire power even if you are small. This Glitch will wear off after a while though.