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)
Continuity mistake: Kim reminds Charlie that they already spent a lot of their savings on fixing up the house to handle more kids, but in the first movie they fixed up the old comic book store because they weren't allowed to watch more than 12 kids in their house by law. (00:08:25)
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.
Continuity mistake: When the children are running out of control and getting ready to get on the bus, Juliette is seen playing around near the bus, then coming out of the woods with another squad. They were supposed to be lost in the woods, so she couldn't have gone to the bus before that.
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