If you place a block right before a piston pushes forward, it will create a ghost of the block. This block will not react to anything, as (I believe) only the client thinks the block is there. if you try to place a block on the ghosted block, the ghost will become the block you tried to place down. It is worth noting that I was not able to replicate this in survival mode. When trying to pull these blocks with sticky pistons, they disappear. 

Not sure if the pictures loaded right so...

Tnt does not react,

Also creates floating sand (also ghosted) 

It seems to be the easiest to reproduce it with sand.
Here is a video showing the bug in action:
<video removed due to autoplay, see attachment instead>