Super Mario Bros. came out almost thirty years ago. You'd think that every glitch and exploit in one of the most popular video games of all time would have been found already. Nope. It looks like someone just found a new one.
As described in a post on old-games enthusiast site RetroCollect, this complicated glitch—which shows up after having beaten the game once already—grants players infinite lives by spawning SMB's magic beanstalk somewhere it's not supposed to be:
First, you must first take player one's Mario to the second level of the game and throw away your first life. With Luigi taking over, player two must traverse all the way to World 5-2 and find the hidden beanstalk block halfway through the stage. Once there, Luigi must start climbing the vines, however, he must await - and take on the chin - an incoming projectile from one of the Hammer Bros. Upon being hit, once player one resumes control of Mario, the beanstalk from World 5-2 will start growing in World 1-2, providing all you need to infinitely kick shells for unlimited bonuses.
Now, this could be an elaborate hoax. But the random nature of the exploit lends it some believability. This infinite lives glitch is so specific and quirky that it seems likes exactly the kind of thing that would get missed in quality assurance testing and millions of random playthroughs by those who've played Super Mario Bros over the years. If any of you retrophiles out there can recreate and document the infinite lives glitch, please share your proof in the comments below.