When you decide to place an item on a packed ice block, for example: button, trapdoor, ladder, or pressure plate, the item is allowed to be placed on it. Meanwhile if you try it with normal ice, it will not work.
MC-48751
Buttons, etc can be placed on packed ice
Resolved
Works As Intended
0
(Unassigned)
Unconfirmed
Minecraft 1.7.4, Minecraft 14w05b, Minecraft 14w06b, Minecraft 14w07a
Retrieved
Packed ice is a solid block.