Tried to investigate (super) yoyo glitch but didn't get anywhere. I learned this much though:
* If you place a hitbox on the ground and an opponent does a strong attack on it (it makes a clank sound), Ness will be repelled briefly. I tried having Ness in various situations while someone attacks his hitbox like that, but nothing special happened. If Ness gets thrown while he's supposed to be repelled, it just freezes him and the thrower for 3 frames and then resumes.
* As you might know, the game remembers the last 12 things that have been hit by one hitbox
Assuming you are in training mode and have your hitbox hit several Mr. Saturn:
Mr Saturn (1) is hit.
Mr Saturn (1) leaves the map.
Another Mr Saturn is summoned, then this one will be (1) now. It cannot be hit.
If two Mr Saturn are summoned, then (1) cannot be hit but (2) can be hit.
You need to summon three Mr Saturn in order to hit a third one, etc.
Basicly, if an Mr Saturn was hit and leaves, a new one can replace it and the hitbox won't "realize" that it's not the same one. You can summon only about 12 items in training mode, but if you use the debug code you can summon pretty much until all slots are used. I'm guessing there are 32 slots. Maybe player characters and stage objects etc. are included in those slots.
There is probably a list for each hitbox that tells what slots have been hit. This list can only carry 12 entries.
I tried finding this in memory but I'm not good at using Cheat Engine and it takes minutes to make one search...
It would have been interesting to see such a list actually update before my eyes so I could understand the conditions for when a hitbox will ignore something or hit something. Super yoyo glitch, as you might know, will just hit the same thing repeatedly. I guess the game fails to put the hit object in the list so the hitbox can hit something every frame or so.
Of course this is assuming Super yoyo glitch isn't a hoax after all.
* The game will treat phantom hits differently from normal hits. Hitboxes will remember if they hit something by phantom hit, so the same hitbox cannot phantomhit a thing multiple times (unfortunately). I'm wondering if phantom hits have their own "list".