Super Mario Bros. is a title that, even after all this time, remains as a central pillar and identifying feature of Nintendo — as a launch title for NES and the game that brought Mario to prominence as a mascot, it's still played and loved to this day. It also helps that it stands up to modern standards in some respects, even if its age shows in the inability to backtrack, along with some rough edges.
That's part of its charm, of course, and keen gamers have spent nearly three decades mastering the title and finding shortcuts and exploits — intentional and otherwise — throughout its worlds. We perhaps thought they'd all been discovered, but it seems another infinite lives trick has been found, though it involves beating the game and embarking on a second playthrough with Mario and Luigi. Below is an excerpt of an explanation from Retro Collect.
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.
You can see this in action below, which appears to be a legitimate method to us — it seems overly elaborate to be faked, though some assist tools can't be entirely ruled out based on the crazy skill on show.
Either way, this is a fun video to watch, though there are far more accessible ways to earn infinite lives in this NES classic.