I was testing my smart furnace setup and got sometimes realy strange Itemcounts. When you smelt 4 Iron/Copper ore, then switch to steel for 1 craft and then back to iron or copper ore, it will give you 3 plates.
Reproduce:
place electric furnace
drop 2 Prod3 modules in
smelt 4 copper ore => get 4 copper plates
smelt 5 iron plates => get 1 steel
smelt 1 copper ore => get 3 copper plates
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If you switch more often through recipies, it will produce even stranger numbers...