In World 8-Castle of Koopa, take the top path at the end of the Donut Lift room. You should be in a room with more Donut Lifts, Bowser statues, and fireballs. Squat and fly (easiest with a P-Wing) and land on top of the last Bowser statue's head. Stand up and Mario will pulled through the wall to another room! It's not really a new room; it's the room you can find by taking the lower route in the Donut Lift room.