In 14w33 and up, FallingSand that is summoned using an execute command relative to a player, is actually summoned relative to the player (e.g. If the player was standing at x=0.5,z=0.5). This didn't used to happen in the snapshots below this, and causes quite a problem.

The problem with this, is if that a block tries to land on a block, but mostly lands on a block that's an airspace, the block will break and fail to solidify.

In the first picture with stone falling down, you can see the FallingSand entities overlap. These blocks will pop, since they land on only a quarter stone, and 3 quarters air, which means that it will break.

In the picture with the sand riding Item frames, it shows how and why it breaks, and how it sometimes prefers to break on one side rather than the other.

EDIT:
Relative summoning apparently doesn't only affect FallingSand, but also every other summonable entity.