Factorio Forums

When I play vanilla Space Age I was used to throw trash ores from inventory to furnaces, and after a while I found that my belts have mixes plates and ores.
After long time I was able to reproduce this bug, and noticed that the furnaces create a new slot when the manipulators have nowhere to put ore.

This video provides a clear sequence of actions to reproduce that bug: https://youtu.be/sjo7wTn_zQc

Copper is a spoilage product of copper bacteria. If an inserter is forced to hold a spoilage product that its target machine can't accept, the product is forced into a spoilage output slot so the inserter doesn't lock up and require manual intervention.