On the topic of furnaces, this is more of a UX thing than a bug, but when smelting many ore and the last fuel runs out, it's not possible to add more fuel until the furnace is stopped (either by completing the smelt, using up the fuel, or by manually stopping the furnace). It would be nice to be able to add more fuel while the thing is still going. Also, it's possible to pull out all the ores/fuel by clicking them when the furnace is in a stopped state, but not in a started state. Perhaps this too could be deliberated further?
Another thing: when smelting, in the instant when fuel is "accepted"/used by the furnace, its count drops by one immediately. Taking out all the fuel still leaves some partial fuel in the furnace which is used up until the next fuel is supplied. This is a fantastic mechanic, and I love this attention to detail.
However, the same does not apply to ore. Partially smelted ore does use fuel, which is correct, but it does not use up the ore. The entire ore can be removed from the furnace even if it has been almost fully smelted. This feels kind of strange; I think the ore should disappear the instant the furnace accepts it. if the furnace is stopped mid-smelt, then the partially smelted ore should either disappear or remain in the furnace like partial fuel until the next ore is accepted.
Maybe above/below the progress bar a small icon could be shown to indicate which fuel/ore is currently in the furnace?