In 2-7 (Lakitu's Wall), eat at least one fat Shy Guy to give Yoshi a huge egg, then get to the part of the level with the Message Block and the car morph bubble item. Shoot a big egg straight up at the Message Block. The egg will hit the block twice, causing the screen to go crazy. Yoshi will disappear, the stars will come to the foreground, and you might even have a small message text box lingering on the screen. Even though Yoshi is gone, you can still move around. You can go back to the left to the previous area to get things back to normal, or, for a real challenge, try finishing the level! The game will freeze up if you go through the goal ring, however.