No Rhythm

Have you noticed how, in most non-musical movies, when there is a dancing scene (again, not in a musical) the actors and extras seem to dance to their own music and not the music that they add when post producing? It irritates me when they don't dance to the rhythm, or the type of music they're playing. Boom, boom, boom, and they go beep, beep, beep. Arthur Murray and Gene Kelly (am I dating myself?) must be turning in their graves