Minecraft 15w34d, Minecraft 15w36c, Minecraft 15w37a, Minecraft 15w38b, Minecraft 15w39c, Minecraft 15w42a
Hoppers can't remove empty bottles left in brewing stands when brewing lingering potions with Dragon's Breath.
To replicate, have a splash potion in the bottom of a brewing stand then put Dragon's Breath in the top. Once it is finished brewing, place/activate a hopper beneath the brewing stand and observe that the hopper will remove the potions but not the empty bottle.
I have a video quickly demonstrating the bug here:
This could be intended behavior, but I doubt it because it contrasts with the hopper's ability to remove empty buckets in furnaces left by using lava buckets as fuel.
This bug is a problem because it prevents automatic brewing machines from making lingering potions. I demonstrate such a broken machine here: