Continuity mistake: Martin covers the girl with a blanket in the "evening" while the boy is chasing the fireflies. It is never cool enough in GA during the season of fireflies to warrant a blanket. Never. Unless there was some weird and unexplained weather phenomenon. (00:56:00)
Revealing mistake: Near the end of the movie, when Malcolm's partner pulls out his gun and Lester shoots him, the bullet hole in the wall appears before the bullet wound in Paul Giamatti's arm.
Answer: I think it's the toilet unclogging. Also, he was probably looking at the diarrhoea she left behind in the toilet bowl.