Minecraft 1.9.3 Pre-Release 1, Minecraft 1.10.2, Minecraft 16w43a, Minecraft 1.12.1, Minecraft 1.12.2
When you press the place/interact key with a slab item on a bottom slab, it tries to place it, even when that action collides with an entity. Meaning: It plays the animation and off-hand items can't be used.
- Slab on floor
- Slab in main hand
- Bow/food/whatever in off-hand
- Stand on slab
- Look down
- Press action key (standard: right click)
→ Bow/food doesn't get triggered, but slab, although it can't be placed.