Affects Version/s: Minecraft 1.5.1, Minecraft 1.5.2, Snapshot 13w19a, Snapshot 13w21a, Minecraft 1.6.1, Minecraft 1.6.2, Minecraft 1.8.1, Minecraft 1.8.3
Fix Version/s: Minecraft 15w40b
Falling Sand (or any gravity-affected block):
-CAN land on these blocks, if it is dropped by your character.
-CAN'T land on these blocks, if it is spawned from a spawner.
-CAN'T land on these blocks, if it is dropped by your character, and you quickly log out and log back in.
-end portal frame
Check gif below for example.