Camera Bug - S4-E10
Revealing mistake: When they wheel the stretcher with the unconscious teenager from the school fire into treatment 3, the stretcher is pushed up against the ER's gurney, and when the teen's right arm gets caught between the stretcher and gurney, the supposedly unconscious teen moves his arm up out of the way. Then he lifts his left hand when the attendant removes the blanket.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy