This one will take a while to explain, but basically we'll be using a variation of the walk through walls glitch to get Mario on the right side of the screen when he passes through the area with the Vine that leads to the Worlds 6/7/8 Warp Zone. This will fake out a nearby pipe, which will take Mario to the warp zone instead of the vine! Here's how it's done...

At the beginning of World 4-2, there is a very bricky area with a group of three Goombas near the end of it. Before the Goombas, you'll come across a small hole in the brick floor. Make sure to drop down and get a Mushroom from the last block in the overhang if you're small -- Mario must be "Super" for this glitch. Then, standing above the hole, jump straight up twice so Mario breaks two Bricks [1].

Next, slowly walk to the right (watch out for the Goombas) until the two-block column you just created has scrolled about half-way off the left side of the screen [2]. Go all the way back to the left side of the screen, then duck and jump once. Mario's feet should automatically "catch" the wall, and he will appear to be stuck [3]. (If Mario's feet don't catch the wall, you haven't scrolled far enough. If Mario breaks a block to the right, you've scrolled too far; just break another one above it and try scrolling again.) Release the Down button, and press the Right button once if you weren't already facing right. Now Mario should be sliding through the brick wall -- do not walk or jump until Mario stops at the right side of the screen. When Mario has stopped [4], jump straight up and to the right until Mario is on top of the bricks.

Now, Mario should be on top of the bricks and all the way to the right of the screen. (Important note: Walking back to the left will move Mario away from the right side of the screen, which may cause the glitch to fail. Try to move forward only.) Staying at the right side of the screen, head to the other side of the pit where the three pipes are. Go down the third pipe [5], and voilà -- Warp Zone! [6] This is the trickiest part because, being so close the right side of the screen, you can't really see what's in front of you. If you need to, you can go back a few steps before continuing on, which will give you a little more space in front. Be careful not to scroll too far back.