In the scene where the monks are slapping themselves in the heads w/ the boards...after a couple times...one of the guys in the back moves the board...it hits his head..but the 'whack' sound is not heard. Then a millisecond later all the other monks hit themselves and get the 'whacking' noise. Since the noise was added in post, and one of the monks was off on his timing and hit himself a few seconds b4 everyone else..you see one monk hit himself silently.