When looking at an entity through a slime block held by an armor stand, the entity doesn't look as though it's behind a slime block. This occurs with slime blocks in both the hand and head slots of an armor stand.

How to reproduce bug:
Open Minecraft in 14w33a, and load a new world.
Type the following command in your chat to summon an armor stand with a slime block on its head and in its hand:

/summon ArmorStand ~ ~ ~ {ShowArms:1,Equipment:[{id:minecraft:slime},{},{},{},{id:minecraft:slime}]}

Spawn entities behind the armor stand, and they will appear as clear as though the slime block weren't there.