My friends and I are experiencing problems with mobs appearing to freeze for one player, but they continue to move for other players. For example, cows will look frozen even while you feed them, but they still elicit hearts and breed without moving. Then, a calf appears with XP several blocks away from the cows you fed. 

The real problem comes in with hostile mobs.  A zombie may appear to be standing several blocks away, frozen, but on someone else's screen, you can see the zombie moving. These hostile mobs can still deal damage when they are *"*invisible," but I can't deal any damage to them.  I've even had "invisible" Creepers explode and kill me.