I have found in some buildings they walk through the stairs. This means the upstairs of these buildings are safe from zombies due to the fact the zombie goes through the stairs and thus can't use them. While your upstairs the zombie walks along the bottom floor under you and through walls etc to do so.

I was at devils castle top floor and 2 pieces of clothing spawned under the floor( 2013-12-30_00001.jpg is a screenshot of this by me) and i was able to go prone and reach out and get the one next to the stairs but in doing so fell to my death. This is my main reason why i want this fixed, a lot of people are going to fall to thee death.