mugg1991 | Mario & Luigi Superstar Saga ~ Have Mario while he is sick @mugg1991 | Uploaded June 2016 | Updated October 2024, 16 hours ago.
Forum post:
tasvideos.org/forum/viewtopic.php?p=436507#436507

The glitching starts at 2:25

When doing wideblock glitch in Little Fungitown and causing the block to disappear, Mario is magically considered to be active so you can make him jump out of his x:0 y:0 position into the room and do some stuff. Not useful though, but interesting.

The game softlocks in the end because Mario in the back has x:0 y:0 and causes Luigi to be stuck to him as soon as he moves, therefore trapping them in the wall.

You can, however, use the green pipe and complete Guffawha Ruins by going through it or using the yellow pipe in Fungitown. If you enter Mario's room where he rests as Luigi, the cutscene where Luigi wakes up Mario will play normally. If you enter Mario's room as Mario, the cutscene will be skipped and you have to enter the throne room (may require glitching through the locked door) to progress the game.