2017 NOTE: This project was posted in 2008 and no longer works. I'll keep it up, but please don't ask about it.
Download this project and look in the sprite list for the cat. It's not there, but you can still drag it around and add scripts/costumes/sounds!! This was a glitch I discovered accidentally, and I don't know exactly how I did it, but it had something to do with pressing cancel quickly in the paint editor after pressing the paint new sprite button.