This is intentional, but still noticeable... At the end of the movie when William Wallace is being beheaded, it is in slow motion and the ax seems to slow down as it reaches his neck. Well, in reality, Gibson wanted the blade to come as close to his neck as possible, (without injury) so told the person to start at his neck and draw the ax up, then the would play the segment in reverse, although you can tell due to the inconsistency; where momentum WOULD HAVE picked up, it slows down.