When attempting to equip a missile into a melee slot and the other melee slot is already occupied, you are prompted to [c]ontinue, [a]bort, or [e]quip to the missile slot instead. The equip prompt is not shown when a missile weapon that is incompatible with the missile is equipped, but if the missile slot is empty, pressing e will still equip it there, resulting in an invalid missile setup. The analogous thing happens when attempting to equip a missile weapon into a melee slot when the other melee slot is already occupied and incompatible missiles are equipped in the missile slot.