Revealing mistake: When they're having a rock war, Henry throws a rock at Beverly. When the rock hits her, it bounces right off her, making it obvious that it was a rubber or bouncy ball, and not a rock.
Revealing mistake: When Henry is about to carve his name into Ben, he kicks Henry and falls back through the fence. As he is rolling his hair is darker than usual, making it obvious that it was a stuntman.
Factual error: During the scene while Georgie is running with his boat, look at the grass, and you can see that there is a good amount of sunlight. This wouldn't make sense to have a lot of sunlight during a big rainstorm. It should have been darker due the shadow of the thunderclouds.
Chosen answer: It's their way of telling Richie to be quiet. To get him to stop talking.