I did not encounter anything major. However, when the zombies surround you from everywhere, it is impossible to move as you cannot shoot them all because the spawn rate is more than the death rate. Therefore, if you are surrounded, you are basically dead. But i mean, that is the whole point of zombie games, so it will be stupid of me to call it a "bug". Although, if more people get a little flustered by it, you can consider having a pushing mechanic where you can slowly push the zombies away in the direction you want to go. Nothing serious though.