When Buffy approaches the shy girl who is disappearing, just after breaking up the fight, the camera shoots from over the girls shoulder. Despite the fact that she is supposed to be invisible her hair can clearly be seen at the left side of the screen. When Buffy also goes to slap the girl you can see her stop her hand a good few inches from the girls face and the slap sound has obviously been inserted afterwards.
At 13:23, when Buffy was at the high school, after she breaks up a fight, she sees a shy girl start to disappear so she slaps her. Only you can clearly see the edge of the girl and she's not all see thru. Also when she slaps her you can see that her hand never touched her, a good couple inches away but of course you still hear the slap sound.