Very famously on the Japanese version of the game you can clip through floors using the Arwing's landing feature. This was patched out of the USA and PAL versions of the game. And for a long time that was the whole story.
The way the glitch works is that the game picks a landing spot for the Arwing and the Arwing will just go to it while ignoring ALL collision checks in the process (I'll talk about the patch in non Japanese versions of the game in the next paragraph). The exploit happens because the game doesn't want the Arwing to abruptly change its movement direction or speed to get to where it needs to go, so it instead slowly transitions from its previous movement speed and direction for a tiny bit. This small amount of player agency is enough to clip through floors if we are boosting (aka increasing our speed) and facing the ground as the game lets us continue moving in that direction, even if the floor is in our way (remember no collision checks during landing). This means we go straight through the floor for a bit. Just let go of the landing button when fully clipped through and your out of bounds.
The Arwing landing clip was lazily patched in one specific case outside of the Japanese version of the game. The way the glitch was patched is seemingly with a small but effective raycast to check only the floor collision of the Arwing. The problem is that it is so small that if we hacked the speed of the Arwing to be faster we can clip through the floor again.
But what about non-floors. In theory we can go through walls and ceilings because ALL collision checks are disabled, and while I've had no success with walls, the ceilings of those looooooong tunnels in Mission 9 (Aparoid Homeworld) can be clipped through. Just boost while pointing it at the ceiling and hold land to land on the floor. Despite some slight changes compared with going though floors, it works the same, and we don't need to walk the whole way through the tunnel. Since the patch only checks for floor collisions, this works on all versions of the game.
I made a LOTAD (aka a very very low optimized TAS) to showcase this in a speedrun context and demonstrate that it is faster than the WR time. Current WR is 2:06 and this is 2:03. (Glitch is shown at 0:15 and 1:42).
