In World 1-2: Tilted Tunnel, there is a secret exit at the end of the level. Reach the part with the red coin ring on a slanted purple crystal. Jump up and over the red ring to find a hidden passage containing some normal coins. Once inside that passage, jump up at the top of the first hill to find another hidden passage made of square bricks. Follow this passage to find a hidden pipe. Exit the pipe to find the red flag for the secret exit to Acorn Plains.