I have been unsuccessful at reproducing the issue manually.
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this/\Boogieman14 wrote:Bricks take 2 stone. There probably already was a stone in the input slot, so it can't insert an iron ore on top of that, so it can't finish creation. Also, the furnace has 'generic' inputs, where the item in the slot determines what's being created, so the inserter can't know it shouldn't insert the iron ore. I'm going to guess this will end up in 'not a bug'
Solution: don't mix inputs on furnaces, stick with coal and only one of the possible ores (at least until you know what you're doing and are aiming for mixed-input smelters, but that's advanced topics)