you know when your player specificly one with a MoveArond script from the tornado twins tutorial gets to a certain height in the -y, when you move around, it messes up and you are "bumping" into colliders you never put there? or controls reverse?
i know it happens when the character gets below a certain point on the global y axis. it also happened in the early versions of unity. is this just a general gaming glitch where the game messes up after a certain point?
How low are we talking? Below a very large point and you will start to get floating point inaccuracies (every place value you go out to the left, you lose one on the right).
Another possibility could be the code itself. I doubt they(aka the tornado twins) would intentionally write code that doesn't work beyond a certain point, or accidentally write code that does that, but without seeing any code I cannot say certainly that there isn't a bug in the code.
I'm also not sure how you got that low, but if you are falling, then the character might exceed a speed at which they can pass through a collider in 1 frame. That's why Unity introduced continuous collision detection. In that case, you have to either limit your character's speed, or use some form of raycasting to ensure that it will not pass through.
P.S. If a "Tornado Twin" views this message, I don't mean to degrade your work in any way. I have a lot of respect for what you guys do helping people new to Unity find the way. I just don't want to rule out something without certainty.