I actually know what happens since i tried it, you use the warp whistle on World 8 it takes you alllllllllllllllllllll the way back to the first warp, meaning warp worlds 1-4..... congratulations you may now proceed to "FUUUUUUUUUUUUUUU......." oh and what makes it worse is that if you have it playing on an emulator it saves states right when you use the warp whistle...... ok that might be a lie.... but it'd be damn funny if that did happen. what makes it worse is that you can't just be like oh well i'll just warp back to world 8 nope since you used the whistle and you only use 2 your stuck in world 5-7.... then your officially fucked.