I have tried to use a invulnerable, inverted, invisible slimes in snapshot to stack a MinecartCommandBlock but they act as normal slimes! Heres the command. Try it in 1.7 (where it functions normally) and than in snapshots!
Command: summon MinecartCommandBlock ~3 ~ ~ {Command:test,Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Slime,Size:-2,Invulnerable:1,ActiveEffects:[

{Id:14,Amplifier:-1,Duration:200000}

],Riding:{id:Zombie,CustomName:Test,CustomNameVisible:1,Equipment:[{},

{id:301}

,

{id:300}

,

{id:299}

,{id:397,Damage:3,tag:{SkullOwner:Srdtrk}}],Attributes:[

{Name:generic.followRange,Base:0,Modifiers:[]}

]}}}}}}}}