This glitch will only work in the Japanese version because the U.S. version has no tall wall near the goal. See the SMB3: From Japanese to English special for more details about this change.

Use a P-Wing before entering World 5-1. After coming out the other side of the pipe that leads to the goal, fly up and over the wall on your left. Travel left on the top of the wall, and you'll hear the treasure-box-appearance sound effect. (You are on the ceiling of the bonus area that you can reach by flying straight up at the beginning of the world.) Go back down, and head for the goal. The Nipper Plant under the Roulette Block has transformed into an object that constantly changes from a "5UP" sprite to a Warp Whistle and back again!

Stranger still, after you hit the Roulette Block, a treasure box will appear! If you hit the Roulette Block with an average jump, the treasure box will appear on the left; if you still have the P-Wing, try to time your tail-flaps in the air so that Mario hits the Roulette Block and continues to fly up a little more, which will cause the treasure box to appear on the right because Mario was flying up instead of moving right (like the treasure boxes in the Wandering Hammer Brother battles, this one appears on the side of the screen that is farthest away from Mario). If you get the treasure box to appear on the right, Mario will collect both the Music Box from the treasure box and the card from the Roulette Block, although the graphics will glitch a little and replace the "Course clear! You got a card" message with scrambled background graphics.