Destroy the 2nd block next to the pipe if you have to, stand on the left of the bottompipe, turn left, duck down, jump and aim to the right, backwards so to say.
If you manage to hit the right spot (this will take some tries) just like in the gif or below the 1st block next to the pipe, but don't destroy that 1st block, the game
pulls you through the wall into the warpzone. The first and the third pipe lead the level -1, the middle pipe takes you to world 5. Take care to not scroll the screen
too far to the right, once the normal warpzone text appears the glitch doesn't work anymore.
Level -1 is already not very legal gameplay, but you can even top that by placing a level end flag into level 8-4 with a ROMhack and make
the game jump to level 8-5 and so on. An additional endflag in level -1 makes you visit level -2 and so on. And as all these levels actually
don't even exist strange things happen, just see for yourself...