I had a bug: As I finished wave 22, there where like 'ghosts' of zombies there. When I walked left or right, they stayed on their position on the screen (they 'hovered'). I could shoot them, but they had no life bar, but they lost flesh when I shot them. On wave 23 the wave bar increased as usual, but stood still at around 15% and from then on no zombies came anymore. I could finish repairing in peace and then I drove off. Odd..