Go to the bell tower with a banana on it, and jump on the small ledge above it. Now, make the camera face inside the tower, and look up into it and you'll see the rocket nozzle that can never be reached! If you have the normal rocket nozzle, you can shoot up with it and see the nozzle box on your way to the bell.
Submitted by: Derek
Edit: It was recently proven by MagicScrumpy that the Unreachable Rocket Nozzle is actually reachable. Please update the information above as soon as possible! Video: https://www.youtube.com/watch?v=-iT_0hPF_98