Steps to reproduce:
- Make a 4x4 block space, with walls all around.
- Spawn a ghast inside of the big hole.
The ghast hitbox fills the 4x4 space properly.
A part of the ghast clips through the ground, and the ghast can't move. Note that if you hit the ghast, it won't go up and fill the space properly, it'll stay there, meaning that there is no free space. That's how I found out that the hitbox is actually offset by about 1 block in the Y axis.