When a fallingsand entity is summoned inside a block, and is given a motion tag, it will make an invisible glitch block, that once you place a block in it, it will change into whatever block was there before the fallingsand was summoned into it.

How to reproduce:
use this command:

summon FallingSand x y z {Time:1,Block:grass,Motion:[0.0,1.0,0.0]}

And make sure that the x y & z coords have a grass block in it.