In certain stages or missions where your hero may earn "bonus experience" (indicated by the green 'pop-up' numbers when the requirement is met), you may press START and choose to exit to the world map; although it is not immediately evident, you keep the XP earned from the "pop-up" value and may replay that same stage to re-earn the bonus again and again. The most easiest way to test this glitch (and get a hang for exploiting it in other campaigns) is Gerald's mission on the Glaucus River. After the successful ambush of the enemy, wait for the green "pop-up" numbers to vanish and choose to exit to the world map, and retry the stage once more.