In the level Gangplank Galley, jump up the first barrels at the start, up where the first Bonus Barrel is. Instead of jumping inside the Bonus Barrel, jump up to the right of it, where the sun is in the background. You'll disappear and hear a sound effect, then you are taken to a "Level Warp!"-Screen. Just go into the right door. You are now directly before the end of this level!