During the scene when Forrest is getting on the bus for his first day of school and he can't find a seat, watch to right before the last boy says "you can't sit here" and look to the right where Jenny is sitting. Right behind Jenny there are two kids sitting and the one closest to the window is a girl in a blue dress. Now watch when Jenny says Forrest can sit with him, and he stares at her. The seat behind Jenny is empty! Now look again when Forrest sits down, the girl is behind him again. Where exactly did she run off to for that 30 some seconds?