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)
Visible crew/equipment: At the end, when Malcolm walks up to testify in the church and the real Big Momma says, "Yeah, maybe," a camera shadow is briefly visible on the right side of Malcolm's back. (01:30:41)
Revealing mistake: When the karate instructor throws the old lady, it’s a stunt double. This is evident because the face briefly changes, and the character’s wrinkles reappear after the fall. (00:43:30)
Factual error: When Malcolm catches fireflies with Trent, they are glowing in broad daylight, which is not possible in reality. Firefly bioluminescence only occurs at night. (00:57:50)
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.