In World 6-1, get a Potion from the first vase. Take it to the grass between two cacti that yields a Potion. Do the illusion item glitch and make sure you get that Potion as the illusion item! Next, enter the cave that has all those vases inside. At this point, your illusion Potion should have turned real by entering the cave. Throw it, enter Sub-Space, then go down one of the vases. You'll warp to 6-1, the same level you were in! Any vase that cannot normally be entered in Sub-Space turns into a backwards Warp Zone with this glitch.