Items of replaceable blocks like grass (tallgrass) or snow layers act like you successfully placed them when you right click the block in the world, even though they were not placed.
Note: This does not happen with double plants because they require a block of air above.
- Place a replaceable block, like for example grass (tallgrass)
- Right click on the grass (tallgrass) block while holding grass (tallgrass)
→ Your arms swings indicating that you successfully placed the block even though you did not
Based on 1.11.2 decompiled using MCP 9.35 rc1
The method net.minecraft.item.ItemBlock.onItemUse(EntityPlayer, World, BlockPos, EnumHand, EnumFacing, float, float, float) returns that the item was successfully used even though it was not successfully placed in the world and therefor not used.