This is an issue that bugs me yet I keep forgetting to mention it.
If you jump into a receding crusher as soon as possible, you die. This is because you land, try to stand up, and are crushed because the standing hitbox is taller than the rolling one.
This shouldn't happen. It's needlessly annoying, and didn't happen in the original games. One of two things should happen:
a) You can't jump into it. This is a poor solution, even if it's what happened in the original games.
b) You tuck into a roll. This is the best solution because if you jump into an advancing crusher, you'll eventually be squished in ball form anyway, and this way low ceiling collision with moving/static sectors is uniformized.