The bug

When you middle click on farmland you get a dirt block despite the fact that farmland is an item as well.

How to reproduce

1. Furrow a dirt block and middle click it
   You will get a dirt block
2. Use the following command

   /give @p farmland
   

   You will have a farmland block

The reason

The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.

The reason why this happens is because the net.minecraft.block.BlockFarmland.func_185473_a(World, BlockPos, IBlockState) method returns a dirt item stack.

public ItemStack func_185473_a(World worldIn, BlockPos pos, IBlockState state)
{
    // Replaced this
    //return new ItemStack(Blocks.dirt);
    return new ItemStack(Blocks.farmland);
}