When you summon a "Marker:1b" armor stand in the absolute middle of a block, you won't be able to press buttons easily inside the block anymore and it's also hard to place blocks behind the block, because the armor stand blocks your view.
In the following video, I have recorded what happens when you try to access a button behind such an armor stand.

In theory, you should never be stopped from pressing the button, because of the hitbox being "0" large.
http://i.gyazo.com/ac5a0dfbb6b9420aa1ad0aa738e0fc90.mp4
This video shows me, constantly (trying) pressing a button by holding "Mouse2" in gamemode 2.
Results are reproducable.
Results occur in gamemode 1 aswell.

I'd like to add the the official use or meaning of "Marker:1b" is setting the hitboxe's size to zero, so the armor stands don't get in your way.
Well, they do and that is definetely not working as intended.