I know this may seem like a joke (as I have posted similar things in the past), but this is a recently discovered way to gain infinite lives in the original Super Mario Bros. that has been dubbed “Violent Buttocks.”
You can jump to the 2:40 point in the video to see the magic.
So here is how the “Violent Buttocks” trick works: After you’ve completed the game, begin a new two-player game in “hard mode.” Play as Mario until level 1-2 where you should die right in the beginning of the stage. Next, you play as Luigi through level 5-2. Here you need to reveal and start climbing the hidden vine in the level, but get killed by a nearby Hammer Bros. during your ascension. After that, you continue to play as Mario. Simply climb up the added bugged vine, smack those two Buzzy Beetles with your buzzy buttocks, and receive a plethora of 1-ups.
Granted it is a lot of work for a reward that is easily obtained in level 3-1, but if do it once you can at least say that you did it once.