Due to the new Zombie spawning system Zombies can be spawned infinitely, and cause major lag issues.
In order to reprodice,
1)Spawn at least 20 zombies, (or gather them if playing survival.)
2)Engage in combat with the group.
3)Watch as zombies fall from the sky, occasionally spawning more zombies.
4)Wait for the sun to rise.
5)Fire damage will cause the zombies to fall from the sky, eventually causing the world to lag to a halt.
(Essentially, the damage caused from falling, burning, and fighting will cause a huge population of zombies to literally fall from the sky.)