The sound of Ji Hoo's harmonica is as if played in an empty dining hall (somewhat spacious) rather than small room filled with people that should deaden its sound.
Ji Hoo's lips don't match with his playing the harmonica.
Ji Hoo drives his grandfather home, only queasy about being alone in the same car with him. It was earlier established that Ji Hoo has a severe phobia against driving cars (due to the childhood trauma of losing his parents in an auto accident, which he fought against only once in a racing bet against Joon Pyo). Here, it should be the Grandpa Yoon (whose presence would exacerbate the trauma), not Ji Hoo (who believes himself guilty of his parents' deaths), who drives them home.