This is a glitch that was originally thought to be a random occurence at the end of 2-2, some people would experience a strange crash and be oblivious as to why. Until someone from #yoshi named ArneTheGreat discovered that it was something to do with the rat carrying an egg. He looked into it further and the most broken category in Yoshi's Island was born. Any% (Glitched).
Getting the Null Egg
Run by the rat carrying an egg and grab the egg, letting the rat fall. Doing this will place the null egg in Yoshi's egg stash as it would any normal egg.
The last sprite to be loaded on the same ID as the null egg will become the new sprite upon entering an area or stage.
Setup for runs <---
Many sprites can crash the game upon entering the goal ring if the glitch setup is done incorrectly.
The game will also crash on many stages when using the boss explosion sprite to complete levels. This is due to the stage being completed before it has been properly loaded. While in theory this would be faster than using the key sprite, it is much more unstable.