I found out that a lot of the zombies are bugging same for my friend.
We were in an house on the 2nd floor when we got randomly hitted while we didn't saw any zombies.. I died my friend survived. When he ran down and got out of the building he saw that the zombie was getting out of an wall of the building and that was the cause of my dead.
Another thing was that I randomly got hitted by an zombie that was 6 meters away from me.. Luckly I had an gun to shoot him but it was still annoying that an zombie could have kill me from an range of 6 meters while me and my friend were looting..