Falling Sand, Mobs, Armor Stands can fall through retracting piston bases. Sand doesn't start falling though.
Falling Sand, Mobs and Players can be pushed into retracting pistons from behind(from the side doesn't work, front can't be tested). I couldn't find a way to push Armor Stands into pistons.

Apart from MC-93631 and MC-93079 it may be related to MC-89262 and MC-94003.