Out of Bounds
Out of Bounds (OoB) is any area where there is no floor triangle directly below Mario. The game cancels any movement that would place Mario OoB. Some glitches can be used to allow Mario to access OoB; this causes him to die instantly and temporarily become hatless. The reason behind Mario losing his hat during an out of bounds death is that Mario forgets to check whether he should have it on, and sets it to its default setting, which is without it[1].
Because floors are carried into Parallel Universes, there are in-bounds regions inside PUs that correspond to the in-bounds areas of the main map. This makes them useful for long-distance movement as in-bounds regions to be in between frames.
Colliding with OoB
When a movement into OoB is cancelled, Mario will appear to collide with OoB. This collision is different from those with walls: Mario's angle is preserved (rather than reflected), and it is impossible to punch/kick or wallkick off OoB.
Reaching OoB
Some glitches can be used to allow the player to reach OoB. Doing so will instantly kill Mario.
- Recovering from a ledge grab into OoB.
- Forcing the only floor below Mario (i.e. a floor triangle of an object) to become intangible.
- Chuckya can throw Mario into OoB.
- Moving OoB by reading a sign clone.
- Warping via teleporter clone.
- Using a door from below the intended height.