Debug mode is your friend. Here's what I've found out so far:
Ceilings in MCZ are one-way: if you're inside a "wall", you'll fall straight down to a road below. In the Chem Plant they're solid.
If you're inside an object, you get shunted off towards the nearest exit. It's possible to be shunted in all four directions. Sides of walls and ceilings count as objects.
Example:
If you're pushed between the wall and the crate here, you get shunted up or down.
If you are standing on something and inside a ceiling without having been crushed, then you can get some zippady-do-da going on. If you try to move left or right, you'll move in the opposite direction at a rate of about really freaking fast. If you don't hit an object you get to keep the speed.
Example:
Sonic is standing on top of one of the spikes. If he moves right, he'll be zipping off to the left at fantastic speeds, where he'll go through a set of spikes and stop inside a wall. If he moves left, he'll be shunted off to the right until stopped by an object (like the vine thing on the right).
And then I was messing around in debug mode; I found that if I went to the top of the level and dropped myself onto the capsule, I died. Apparently I fell through the capsule.