In Super Mario Bros, the screen automatically starts scrolling once you are past X position 112. However, a strategy employed by many speedrunners of the game during level 4-2 is the "backwards bump method", which can push you forward several pixels.
This image is of 4-2, and uses a ROM hack that displays Mario's X position where the time usually is. Notice that it moves up and then locks at 112, even though Mario is still clearly moving forward. That's because the game was programmed to start scrolling once Mario's X position reaches 112. But watch what happens in the next image:
This image uses the same ROM hack as before, and is also of 4-2, but this time, Mario's X position spikes to 120. What the player is doing is he's running into a wall, briefly tapping left to turn around, and brushing on the side of the wall while jumping to get over it. Normally you will move between 7 and 10 pixels forward, and the reason speedrunners use this is to access the wrong warp later in the level.
But why does this work? Why does brushing on a wall while facing the opposite direction push Mario forward several pixels? And why does it push Mario forward between 7 and 10?