This silly mistake was a miscalculation in how many steps the girl "Sara" in the movie was taking. Go to the part when the gang is taken to Dawson's garage by the sweet college guy Dan. Sara and Chris get out of his jeep and start walking towards the garage. Notice when she is walking up the stairs. When she gets to the top she takes another step, thinking there is another stair. It is no big mistake, just funny to watch.