When using /replaceitem or trying to directly summon a mob with equipment in the head slot, all "rotated" blocks show the "error" texture.
Command to recreate issue:
/entitydata @e[type=ArmorStand] {Equipment:[{id:log,Damage:4},{},{},{},{id:stone_stairs,Damage:1}]}
This affects all blocks that have damage values that are a rotation of another damage value, including, but not limited to:
log (damage value 4+)
log2 (damage value 2+)
stairs (damage value 1+)
quartz_block (damage values 3+)
rail variants (damage 1+)
piston variants (damage 1+)
wooden_slab (damage 6+)
stone_slab (damage 8+)
Blocks rendered on entities' heads are not blocks, they are items, just like the blocks in your inventory. "Item" blocks don't have any rotation. I'd argue this is working as intended.