Revealing mistake: In the opening scene, the little girl pulls the cake over to her with only a few fingers which is near impossible. When the camera angle shifts and she lets go of the cake, it still wiggles as if she's pulling on it, showing that it's being controlled from the other side, most likely by an off-camera crew member. (00:01:25)
Revealing mistake: When all the kids shoot arrows at Charlie, you can see one bounce off his hand and wiggle a bit and leave no mark.
Answer: Earlier in the film, when the guys were talking to the banker about getting the camp ready, the plumber came out and said the bathroom had a methane problem and that it needed to be pumped (and a new septic tank and plumbing). The backed up tank is what caused the methane to build up in the bathroom. Methane is highly flammable and will cause an explosion if the gas mixture is right and there's a spark or flame (mines have exploded because of methane build up). When the lights went out, Phil lit the lighter to help him see in the dark better (although we don't really know what his full intention were, e.g. was he going to go see who turned the lights off, wipe, or just continue reading).
Bishop73