When you summon an invisible armor stand, you won't be able to do things through him.

What I expected to happen was...:
The invisible option of the armor stand would also "remove" his hitbox.

What actually happened was...:
I can't break a block, hit an entity or anything else through this invisible armor stand - baby or not.

Steps to Reproduce:
1. Place a command block with this command: summon ArmorStand ~ ~1 ~ {CustomName:"<Invisible Friend>",CustomNameVisible:1,Invisible:1,Small:1,NoGravity:1,Riding:{id:ArmorStand,CustomName:"Jean",CustomNameVisible:1}}

2. Now you have a little invisible friend riding Jean, our visible armor stand. Try to break Jean through our invisible friend : you can't.

3. Try to break Jean from where you won't hit the invisible armor stand : you can.

4. Try to break a block through our invisible friend : again, you can't. And not only through the nametag : through where he should be.

I don't really know if it's a bug, but I don't think this is intended, as the only way to make flying nametags - as far as I know at least - is with invisible armor stand in 1.8.x.