This is a glitch in Super Mario Advance 3: Yoshi's Island. It will NOT work in the original, as it has a different pause screen. To do it, go to the level with those flimsy bridges you can break with a ground pound (I think it's in world 3, not sure). Go to the second one (The one with no land above the water) and ground pound. The SECOND you hit the bridge, pause! You will see the broken bridge fragments start to come from the top of the screen. Unpause, and you instantly die.
Why it Works Edit
Since the bridge is above water, the pause stops you before you are able to start floating. The bridge fragments have nowhere to go, so they come off the top. Yoshi continues to fall, and the game kills him off.
Found on TMK