In the last dance scene, Patrick Swayze jumps down from the stage and starts dancing. He goes down on his knees and when he lifts one knee, it is extremely dusty, however when he stands up his knees are completely clean.
In the scene where Johnny and Baby are practicing dancing, and she keeps laughing when he runs his arm down hers, it was not part of the scene, she was actually laughing and his frustration was genuine. They left it because it was effective. Her falling over in this scene was unplanned too.