At the beginning of the scene where Meg decides that she must be the one to go back to get Charles Wallace, her hair is pulled back into a loop bun with only a few hairs loose (at 01:40:41 as she speaks with her father). In one of the shots in the same scene, it is still a loop bun, but there's a considerable amount of hair that is out (at 01:40:58 after her Emily Dickinson quote), and then in the shot right after that, it's back to being neat with only a few hairs out.