it is true that you expect a defender to be able to clear it, but as was said if buffon had a finger tip on it which deflected the path of the ball it might well be that bonucci would have cleared it easily with the original path of the ball, but buffon deflected it so the ball hit his foot awkward and it went in. this shit happens all the time. bonucci was just unlucky to be there at that time. if you allow an opponent to have an shot on goal which does already beat the keeper you can not blame the guy who tries to make up for two other errors (first: letting the opponent shoot, second: failing to punch the ball away from the keeper).