Put the summary of the bug you're having here

What I expected to happen was...:
I can transport mobs across my bridge

What actually happened was...:
The mob just stayed at the end of my bridge and tried to get around it

Steps to Reproduce:
1. Place slabs with the underhalf submerged (becomes waterlogged)
2. (Just to make it more noticeable) place fences on each side of the bridge
3. Spawn mob on one side of bridge                                                                                              4. Attract mob - tested with Skeletons, Zombies and Cows