- You have a factory producing some item which needs more than half the stack size of an item for the recipe
- You fill the factory via a chest which has 3 stacks of the needed item
- The item is not the last one in the recipe
- Your the needed amount of items to fill the factory after producing an item is not a multiple of your inserter stack size bonus
Solution: The inserter should not try to insert more than 1 stack of any item into a factory