When summoning a mob the world freezes

When I was summoning mobs with the /summon command my world freezes but I can still move around. If I wrote something in the chat it disappeared and i couldn't interact with anything in the world. When I tried to log off the world it didn't work so I was forced to force quit Minecraft. When I later loggen in to the world it had been restored to a few seconds before I put in the command in the commandblock. I changed version to 1.8.1-pre4 but the same thing would happen. I'm on a MacBook Air. My friend, (also on a MacBook Air) tried this and his world did the same thing. These are some of the commands I tried:

/summon Zombie ~0 ~1 ~0 {Equipment:[

{id:wooden_sword}

,{},{},{},

{id:leather_helmet}

],CustomName:Zombie,CustomNameVisible:1,Attributes:[

{Name:generic.followRange,Base:999}

],CanBreakDoors:1}

/summon Zombie ~0 ~1 ~0 {Attributes:[

{Name:generic.followRange,Base:999}

]}

/summon Zombie ~0 ~1 ~0 {Attributes:[

{Name:generic.followRange,Base:999}

],CanBreakDoors:1}

Thanks to: Google Translate for some of the words