Plot hole: Foster served in the military and even received a Purple Heart as indicated by the plaque in his mother's home. Yet he needs a kid to explain to him how to handle an M72 rocket launcher. A weapon introduced in the 60s and thus well within his time at the military.
Plot hole: After the cop is shot, Prendergast heads directly for the pier. There is nothing in the movie that would tell him that Bill Foster would have gone in that direction. (01:31:55 - 01:33:41)
Chosen answer: Because the beggar thought there would be items of value in the briefcase and he was frustrated that there was nothing in there. So in frustration he threw the apple at D-Fens.
pierpp