The zombie crawler is not stopped by the navmesh at all times, the zombie can crouch trough the wall and crouch to you with his body 85% below the floor surface. He cant deal any damage from what I've noticed, he can only follow you where you go.

The other issue is that if you manages to escape a smaller horde of zombies (3 - 5) and one of these zombies is a crawler, the crawler can push a normal running zombie through the door / wall and get to you.

Also, the Crawler is the only zombie I've encoutered where you can be on the upper floor and the Crawler will be on the first floor and he can hit you (such as if the server thought I was standing on the first floor, and therefor the zombie could hit me). You must exit the house and wait for the crawler to exit aswell to kill him.