When setting a block to block id 36, then dropping FallingSand ontop of it with the TileID of 36 you can't break that block. If you try and break the block underneath then the FallingSand tries falling through and crashes Minecraft and gets undone.
Command in top command block: /summon FallingSand ~ ~-2 ~

Command in bottom command block: /setblock ~ ~-3 ~ 36
Pictures: http://imgur.com/vOvmFQU,D17xECQ,bru4wor
In the hole no block can be placed and when looking through that place no blocks can be broken.