Right after Princess Leia records the message for General Kenobi, she leaves & R2-D2 starts rolling towards the camera. But he isn't rolling straight. He is actually rolling towards to door frame. He gets within a foot or so from it then it cuts to C-3PO. In the making of Star Wars, they actually show R2 hit the door frame but it cuts too quickly in the movie to actually see him hit. But you know he had to because he gets too close to do anything but.