To access this warp, you must be Super or Fiery Mario. Go to the pipe at the end of the level and jump on top of it. Jump up and break the block over top of the part of the pipe farthest left. Then, position Mario on the edge of the pipe, with one foot hanging off the edge. Duck, then jump backwards as far right as possible. If done just right, Mario will slide through the pipe and the bricks. Walk over to the warp zone area and enter the middle pipe. This pipe will take you to world 5, level 1. The same area can be acessed from a pipe in the same place in 4-2. However, this is meant to be a warp zone to 5-1, whereas the 1-2 warp to 5-1 is a glitch. After completing the jump trick in level 1-2, you can also go down either of the other two pipes, both of which will take you to Minus World -1.