And by "not behaving properly", I don't mean they are rude.

I mean zombies randomly stuck in position when shooting them (which is convenient but not the expected behaviour), zombies running at 0.1km/h or on position, etc.

Another issue I had which might be related to server synch is about zombies going through closed doors. The situation is the following :

1. Run with a zombie following you
2. Enter a building and immediately shut the door
3. The zombie may be going through the door

My guess is that the server effectively closed the door after a delay, due to player2server synch.