For the zombies moving through walls you might try having them destroy the obstacles as they move, that would get rid of all the complaints about ghosting. It would also let you hide objects in walls, something that would make people have to use more strategy to force the zombies to go where they wanted to.
During the game I found 2 bugs:
When I had a 81% headshot skill and the vets ability to increase haedshots by 20% for each one in a row I found that even right after I got a headshot on one zombie I could still miss (I should have had a 101% chance to hit then).
When I was level 20 I kept gaining exp, and when I hit the max amount it told me I had reached level 20 again and gave me a 21st skill point. I finished the game before I could see if you can reach level 22, but it seems likely you can. I doubt this bug would create many problems though, I pretty much had to clear evey map (twice for each of the maps the maps that regenerate zombies) and trash most items I found to reach this level by the 2nd to last map in the game .