a player or hopper can put a non smelt-able block or item into the smelting slot, this is inconsistent behavior as the fuel slot only allows you to place burnable fuels in the slot.

this makes the use of hoppers with furnaces quite frustrating as you can optimize the fuel supply using a non-burnable non-stackable item like a stone sword to improve the systems efficiency, it would be preferable if you could do the same thing with the ore delivery as it would make such a system more efficient, attached is an example of the system to which i refer