In World 1-2, go along until you see some moving platforms. Get on them and use a running jump to get to the small brick ceiling. Jump across the gap to the right and keep going until you see two pipes. Use the left pipe to find an area with lava. Get small with your character (in other words, get hurt if you've picked up a power-up) and go across the small lava pit under the brick wall. Keep going past there to a pipe. Ignore said pipe, but jump around until you find the hidden coin block nearby. Jump onto it and step carefully to its right edge and jump: you should find another block. Jump on this and up to the ceiling, then continue along to the warp zone.