The the episode "The Storm" when the children can't pay for the food in the market, the Melon Lady kicks Sokka on the left side as he is walking away and he starts to rub the bruise with his left hand as he is walking away from the camera. In the very next shot, we see him from the front and he is now rubbing the bruise on his right side with his right hand. The bruise must have moved between the shots.