While messing about with the summon/fill command(s), I had a fenced in area with mobs within it. I used a fill command with the x/y values just beyond the fence posts, from a few blocks above. I noticed when I went to break any sand block that landed directly on the fence post itself, I couldn't. Almost like a hologram. The only way to remedy this is to break the fence post to let the sand fall all the way to the ground level. I will show two comparison screenshots to show what I mean. The first screenshot is the same setup(I placed each block onto the top of the fence post, adding one to the second post) so that I could break the bottom block and allow the top to fall. And the second screenshot pretty much explains what happens. The block on the bottom breaks on the first post no problem. As it does on the second post, however, the block sitting above that bottom block falls to a certain height above the post and stays there like an unbreakable hologram(can walk through it, punch through it etc...) Break the post below it and it lands on the ground and returns to normalcy(screenshot 3).