When a FallingSand entity is summoned at the same coordinates as the command block that summoned it, the command block becomes invisible, however, you can not walk through it, if you had a button on it before, the button is still there. You can not walk through the block or chop it, you simply chop through it. If you click the button, it will execute the command like a normal command block. If a block is placed in the space where the command block is, it will show the block placed for an instant, then show the command block, an it will work as normal until another FallingSand entity is summoned at it's coordinates.