SMS - Uninitialized Variable Glitches

Sirena 1 Instamanta
See https://pastebin.com/DjPSjtNc

Sirena 5 Laugh Skip
The object that detects Mario going under King Boo copies its X and Z coordinates from a matrix in King Boo's model, but said matrix only gets initialized once King Boo first appears. As a result, if the values in memory don't work out, there's no way to make King Boo appear earlier than normal, as the trigger only gets to its usual location once King Boo is already there. Alternatively, the values can work out in such a way that the trigger is in Mario's way as the roulette brings him down at the start of the fight. This causes King Boo to hide during the cutscene, the roulette to stay in mid-air, and the fight to become very weird as anything King Boo spits out will immediately die for being out of bounds.

Pianta 1 Unaware Chomplets
When a Chomplet successfully attacks Mario, it becomes unable to notice him for a few seconds. This is done using a timer field which is set to some value once Mario gets hit and decreases every QF until it reaches zero. Until then the Chomplet will not run its normal distance and angle checks to notice Mario. But the timer isn't set to 0 when the Chomplet object is constructed, meaning it could take up to a few billion QF before a Chomplet is able to notice Mario.
Note that spraying a Chomplet with a nonzero timer will still make it aggressive, and getting hit by an aggressive Chomplet will reset its timer to the normal cooldown, ending the glitch.

Pianta 2 Text Skip
When Piantissimo finishes the race and Mario is close to him, a script makes Mario talk to the nearest NPC, but that NPC isn't necessarily Piantissimo himself; if Mario is close to both Piantissimo and the nearby Pianta lady, he'll talk to both, and both will try to show their text. Which one "wins" depends on a glitch in the script interpreter: a seemingly unused mode limits the number of instructions to be run on every call to the interpreter, but whether that mode is enabled or not depends on an uninitialized field. If the field is 0, Piantissimo sets his text first, then the lady overwrites it, leading to the text skip. Otherwise the opposite happens.

Pianta 5 Reds Fast Textbox
Pianta 5 reds speedruns involve ground pounding the red coin switch, then hovering from platform to platform up to the yellow Chuckster, which is required for reaching the upper section of the level. Talking to that Chuckster sometimes triggers the same sound as the red coin switch, and the text appears much faster than it should. This is caused by the script interpreter glitch mentioned above: the script responsible for NPC dialogue takes more than 32 instructions to check for the correct NPC and set the corresponding textbox properties, so when the glitch is active, the properties aren't set until the next QF. If Mario is facing straight towards the Chuckster (i.e. takes no time to turn towards him), the textbox will read the properties before the script has time to write them, so the last displayed textbox's properties (the red coin switch's) will be reused. This can be abused in Piantissimo levels by reading signs, which doesn't lose time as Piantissimo keeps running. In particular, it can be combined with the aforementioned text skip in Pianta 2.

Pachinko Fake Spins
In some area/version combinations (like JP 1.0 pachinko), it's possible to spinjump without inputting a whole spin: https://clips.twitch.tv/ConcernedDirtyCrowPeteZaroll-9kbDQVzo43Vixa6D
When checking for spins, the game goes over the stick inputs from the past 10 frames and for each, check if it points towards the north, west, south or east. It keeps a separate variable for each quadrant, which is set to 1 when a matching value is found. Once all the inputs are processed, it checks if all 4 variables are equal to 1, and registers a spin if that's the case. But since the variables are uninitialized, they may happen to be equal to 1 from the start, in which case they will stay that way and count as if the corresponding directions were matched. In JP 1.0 pachinko, the north and east variables are already 1, so holding the stick south and west is enough to trigger a spinjump.