The bug
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.
How to reproduce
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.
Code analysis
Code analysis by @unknown can be found in this comment.
Linked issues
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Please link to this comment in the description
The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
The reason why this happens is because the method net.minecraft.item.ItemSlab.onItemUse(ItemStack, EntityPlayer, World, BlockPos, EnumHand, EnumFacing, float, float, float) returns that it sucessfully used the slab if the slab matches with an existing slab at this position. Instead it should only return that the slab was sucessfully placed if it was really placed. Same applies to the method net.minecraft.item.ItemSlab.tryPlace(EntityPlayer, ItemStack, World, BlockPos, Object).
Attached a screenshot with bow in offhand and slab in main hand playing the animation. Notice that my hands are set to left-handed mode.