This is near the end of the movie. In the part where Ferris has just run home, and Mr. Rooney is at the back porch with him, Ferris' hair is messed up, and his mouth is open and he's breathing hard. But when Jeannie looks at them through the window, his hair is neat and his mouth is closed. The scene keeps switching back and forth, and his mouth is open, then closed, then open... It's kind of funny if you notice it.