Breaking placed signs on blocks in specific directions gives different wooden plank particles.

Breaking placed sign (facing North) = Acacia texture.
Breaking placed sign (facing East) = Birch texture.
Breaking placed sign (facing South) = Dark Oak texture.
Breaking placed sign (facing West) = Jungle texture.

Note I've seen this happen earlier in 0.15.0 (perhaps earlier), so this has been around, and it's still happening now.