Audio problem: After killing a skeleton, its bones will fall to the ground and rattle around making loud noise. However the radgoll and physics for this was never finished correctly, and thus even after the bones stop moving, they continue to make the loud rattle noises until they vanish, which takes a while. And the bones tend not to stop moving either. This glitch is also present in NZA2.
Revealing mistake: If you get a slow motion bullet cam shot on a zombie wearing a helmet or hat, and it's a head shot, you can see the hat fly off when the bullet hits, but often you will see the layers of the hat pass through the zombie's head with no collision.
Trivia: 'City of Ashes': in the hospital scene, there is a board on the wall with the names Dr J. Mengele, Dr W. Heisenberg and Dr G. Rasputin listed, referring to the infamous Nazi-German doctor/anthropologist, Joseph Mengele, German nuclear scientist, Werner Heisenberg, and Russian monk/witchdoctor, Grigorij Rasputin.