When the boy accidentally hits the old man with the ball, he falls over. Note that you can see the stuntman's full head of hair as he hits the ground. When you return to the close-ups of the old man as he gets up, that extra hair is gone.
In the country where the film was made, extras are not hired. The people in the background are just normal people. Watch the background during the scene where the young couple is talking in front of a subway staircase and you will see a boy flipping off the camera.