If you have a Tanooki Suit saved from World 4, put it on for the World 5-2 start area. Position Mario near the top of the slope, then jump straight up. While in the air, press B while holding Down to turn into a statue. Statue Mario will slide down the slope and get "squashed" when he hits the left side of the screen. Normally, a moving Mario is stopped when the screen won't scroll to the left, but a sliding Statue Mario can go outside the play area.

You can also reproduce this glitch in World 7-3 if you start on the highest point of the slope.