It's amazing that nearly 30 years later people are still finding interesting glitches and tricks in the original Super Mario Bros.
Here's the latest, from Japanese YouTube channel Games Haya (check out the rest of the channel, incidentally, for a bunch of other ridiculous Super Mario Bros tricks).
To do the trick, you need to have already finished the game once. When you play for the second time, all the Goombas will be replaced by Buzzy Beetles.
Then, you have to play a two-player game. Get player one to level 1-2 and player two to level 5-2. Then kill player two as he's climbing up the hidden beanstalk and, well, you can see the rest below.
You can skip to 2:30 to avoid the build-up but it's worth seeing if you have time because this guy's a master. He even does the minus world glitch at the end of World 1-2 to reach World 5 quicker.