In this short video, you will see all about the level 4-2 one of the most famous in-game, and how by using some very hard to play tricks speed runners were able to trick the game into thinking that Mario was several frames forward of where he actually is. Normally in the game, Mario is 112 pixels across the screen but by hitting into different objects in a particular way it could move players farther along the level. The reason this is important is because back then when the game was developed it only allowed one warp zone to be loaded into the game at once, and 4-2 was different because it had a part of the level with 2 different warp zones back to back. By planning the position of Mario in the level players could actually glitch themselves into the fast warp zone by using these methods.
Check out the video below, it is quite interesting.