Find the Ice Blocks in Worlds 3-9 or 7-5 that are placed up against a pipe. Kick away some Ice Blocks so that there is one remaining on the right side of the pipe, with one space between the block and the top of the pipe. Next, position "big" Mario underneath that block [screenshot 1]. Do a ducking jump, and then hold Left (while still holding the Down button) in mid-air. When Mario hits the Ice Block, he'll bounce between the block and the top of the pipe for a short time [screenshot 2], and then he'll go down the side of the pipe [screenshot 3]! After the screen scrolls, Mario won't come out of the pipe and will appear to be stuck behind the background [screenshot 4]. From here, you can move left and right to scroll the screen, but there is no way out. If you're Fiery Mario, shoot some fireballs and they will appear to come from different places each time.

The bad news: When time runs out, you'll have to wait a few minutes for the game to return you to the map screen.