Bug
Pretty good game, but I ran across a bug. On the minimap on wave 13, it says there's one more zombie left in the top left corner of the map (bottom right corner of the screen), but I couldn't find it.
Not sure if there's a set number of zombies on each wave, but in those 13 waves, I ended up with 427 counted. So if #428 is a bug or not, I thought I'd let you know.