Signs, Item Frames, Redstone, etc. shouldn't be able to be placed on transparent blocks. For some reason, barriers, which are the most transparent blocks in the game, allow you to place these blocks on them.
MC-61993
Placing items that require an opaque support block on barriers
Resolved
Works As Intended
0
Legofan Minifig
(Unassigned)
Unconfirmed
Minecraft 14w28b
Retrieved
Its intentional because its a block without additional data.