Some entities (such as villagers) get stuck on waterlogged bottom slabs

When certain mobs walk onto waterlogged bottom slabs, they will just stand there and never move. In the case of villagers, even if there is a zombie nearby, they won't run away. 

HOW TO REPLICATE:

1.) Lure a mob onto a waterlogged bottom slab.

2.) Wait... it won't leave that block unless pushed out.