suppermariobroth:

In Level 6-9 of Super Mario Bros. 3, it is possible to skip the entirety of the level by using a glitch to jump over the wall at the start. If Small Mario jumps towards the wall, touches it at exactly the right pixel and the A button is pressed again, he will perform another jump in mid-air and have enough height to get to the top of the wall. From there, going to the right will lead directly to the end of the level. Warning: there is an unseen Koopa Troopa on top of the wall that may kill Mario, so it is best to keep jumping to avoid touching it. (Source)