What I expected to happen was...:
I had a mob grinder with a hopper directly below the collection point where the mobs would take fall damage. The shadow of the mob rendered below the hopper. I noticed that this works with any transparent block that you can see part of the under half of the block.

What actually happened was...:
The mob shadow would render below the transparent block instead of not rendering like it should have.

Steps to Reproduce:
1. Build a small box like in the picture below (make sure you use an upside-down slab), and you will find that a shadow will render under the zombie.