Continuity mistake: When the old man is fixing the water fountain, a towel appears / disappears between shots.
Continuity mistake: After water falls on Billy's desk, the table and floor appear totally dry.
Continuity mistake: When Billy is tied up in the dentist's chair, he's got a single wire tying him up, but a double wire in the close-ups. The bandage on his forehead is also either covering his eyebrows or not, depending on the angle.
Answer: He wasn't relevant to the plot.
Rob245