What I was attempting: To arrange two chests in a way to allow for easy loading of a furnace.

What I was expecting: All of the materials should have transferred to the upper slot in the furnace.

What actually happened: The materials split up between the adjacent hoppers, and only half of the material ended up in the upper slot, while the other half clogged the fuel hopper.

When arranging three hoppers above and to the side of a furnace to enable quickly loading ore and coal, I put a trapped chest on the top of the hopper arrangement leading to the side of the furnace, and a regular chest on top of the hopper that lead into the top of the chest. If I put any ore/cobblestone/etc. into the regular chest, it is also distributed into the hoppers that are below the trapped chest, thus splitting the stack inserted into the regular chest in half. This does not happen if the trapped chest is on the left side, and the regular chest is on the right. See the image for a better visual description.