Suckage glitch
The suckage glitch is a glitch that allows the physics of the boundary surrounding the Metro Kingdom to be abused to send Mario flying to distant locations.
Explanation
The entirety of the Metro Kingdom is surrounded by an invisible barrier that behaves as a force field. Rather than simply stopping Mario when he touches it, as a regular wall would, it applies a force to Mario if he is outside its limits, and this force gets stronger the farther Mario is outside the boundary.
The interior of the New Donk City Hall Auditorium is actually located in the same scene as the mainland of the Metro Kingdom. When Mario enters the auditorium, the game simply moves Mario to a distant section of the map where the interior is located. However, this location is actually outside the kingdom boundary, so in order to prevent the force pushing Mario back toward the mainland, the game temporarily disables the force field when Mario enters the loading zone to the auditorium, and re-enables it when Mario walks into the exit loading zone from the inside.
However, it is possible to reach the interior of the auditorium without entering the loading zone. This can be done by exploiting the mechanics of the Assist Mode bubble. First, the player must enter the auditorium normally, and walk to the back of it to place the bubble's safe spot. Then, one must enable two-player mode, and bounce on the New Donkers to exit the auditorium without moving the safe spot. Upon exiting, the player must press A or B on Cappy's controller to bounce Mario at the exact moment that Mario becomes controllable; this prevents the safe spot from being placed at the spot where Mario walks out. Mario must grab a ledge and shimmy over to the edge of the kingdom to fall into the void.
Another way to prevent the safe spot from being placed as Mario walks out is to use the Letter Escape glitch to place a letter right where Mario exits, and use two-player mode to capture it before Mario is controllable.
If everything was done correctly, Mario will be put into a bubble and carried all the way to the distant location of the auditorium's interior. Mario will now be pinned to the right side of the room, since the player never entered the loading zone to disable the force field.
In the back right of the auditorium, there is a spot where Mario can clip out just by doing certain movements (this has been observed with crouching[1], rolling[2] and ground pounding[3]). Essentially, the force exerted by the force field will outweigh the force of the walls keeping Mario in bounds for a brief moment, causing Mario to clip through and be sent flying a great distance back toward the mainland.