Mod version: 3.3.5
Minecraft version: 1.7.10

Circumstances under which behavior is detected:
Garage door is 3 blocks in height. When open, garage door takes up 1/2-block height at the top of the three-block-high space.

Expected behavior:
Zombies (even baby zombies), creepers, and skeletons should be able to path through this space.

Observed behavior:
Zombies (even baby zombies), creepers, and skeletons cannot path through this space.

This behavior is contrary to:
Spiders can path through a two-block tall garage door, provided it is also at least two blocks wide.

I find it interesting that it works with spiders. I'll make a note to check why this is the case and see if it leads me anywhere that could be helpful.

I believe it is because it appears as as solid wall to them. Spiders path directly for you regardless of what's in the way. Should be the same for slimes (haven't tested, but it makes sense). Ethereal Glass (from Extra Utils) has a similar effect on pathing.

Skeletons will still shoot you through the door, but if you move away in such a way that they can't adjust to shoot you, they will go looking for an alternate entrance.

IF you can figure out what makes it appear as a wall, that might help for making Carpenter's Fences and Gates seem like solid objects to mobs as well - currently they can't "see" them.