And for the players stuck in the ceilings, I think it has something related with the crouch / un-crouch engine physics, it's pretty easy to do, a lot of players have fun with it, but get ready to stay blocked all round if there are no admins to unstuck you.
It also seems to me that if a player who touches the ceilling with his head, gets infected, it can happen that he is blocked because the zombie skin must be larger than that of humans, so takes more space, but it takes place in few fractions of second with the engine as well