On occasion, I pick up a Ship Shield Battery, or other consumable item in space that will not stack on existing Like Items. If I try to stack them, they simply Swap Spaces in the Cargo Panel.
Then there is the Kit with Combat Consumables. They also do not stack with existing like items in the cargo Panel. The kit creates one each Hypo, Power Cell, and Shield Charge. Each takes its own space in cargo Panel, and will not stack with existing units already there, that may have been gathered as drops.