When you try to throw grenades (chemlights and smoke or fraggrenades, NOT IR) through the empty space of Shoothouse walls (eg. windows or above crouch/prone/stand walls) it will collide with the object.
The space this is happening seems to be as big, as the full version of the object.
Tunnels seem to work fine, exept for a little space in the mid.

https://youtu.be/jNAvvQnM0J8
left side normal, right side simple objects