After defeating the Abyssal henchmen, the Lucario comes to protect the Riolu and Mega-Evolves to do so. Following this, the henchmen say they'd better run, and the screen goes dark, but when the picture comes back the henchmen are still present. Then there's a little dialogue between Lucario and Nora, after which Lucario moves away and the game freezes. Music is still playing, but the player character, Nora, and the Abyssal members are motionless on the screen.
I feel this is pretty major because it's a bug that prevents any further progression through the story.