The cobblestone wall does not render correctly.
EDIT: It only happens when connecting to solid blocks (I think).
MC-84735
Z-Fighting at top of Cobblestone Wall
Resolved
Fixed
2
Confirmed
Minecraft 15w31c, Minecraft 15w32c, Minecraft 15w33b, Minecraft 15w38a, Minecraft 15w43c
Minecraft 15w44a
Retrieved
Confirmed. Happens for all the blocks using connecting models in some texture packs (very annoying for redstone wire).
In vanilla the effect is noticeable all along cobblestone walls, though most visible when connected to a block indeed.