Six of the nine doors In World 3's first fortress lead to a floating door on the other side, dropping you into a pool of water that leads back to the pipe near the beginning of the stage. But if you press Up again right before Mario appears in front of the floating door, you'll end up entering it, which will warp you to another part of the fortress. Door numbers one, two, seven, and eight will warp you to the gray pipe near the beginning of the stage (Mario will end up overlapping the pipe, but he'll slide out safely). Door numbers three and four will warp you to Boom-Boom's room!

The exact timing is to press Up 50 frames (5/6 of a second) after pressing Up the first time.