Bugs:
When you're fighting a boss and use a few bullets, and then die, you don't get the bullets you had before the boss fight began. This quickly makes the boss unbeatable. Also, a flaw in the game is that the bosses have WAAAAAAAAAAAAAY to many invincibility frames, becoming immune for 3 seconds at a time. The second boss also does WAY to much knock back and damage when they're together. Lower the knock back or the damage.
ONe of the more realistic zombie games I've seen, the game ends as soon as the army arrives, sure it took ridiculously long for them to get there, but still, the zombies wouldn't be able to handle the military.