Deliberate mistake: After Johnny's and Captain Stone's efforts, when the construction site's cardiac arrest is wheeled into Rampart, the problem is twofold; First, we hear Roy's voice saying, "Full arrest, number four," even though Roy is not actually on this run (that line is from S4xE21 "Back-Up", when they wheel the cardiac arrest into treatment 1, after defibbing him atop Engine 51). And second, the attendant pulling the stretcher changes, and the EKG monitor lying on the patient is the old blue Datascope 850 from earlier seasons (this shot is from S4xE7 "Daisy's Pick", the hypothermic victim - note his wrapped foot!), whereas at the construction site and when he's wheeled into the treatment room, the EKG monitor is the newer white Datascope M/D2.
Deliberate mistake: After the fireworks begin exploding at the warehouse fire, when Captain Stanley and Roy are helping Willy out, they are knocked to the ground by an explosion, and in two of the following exterior shots the large sign that reads "Kiddytime Toy Co" is visible. This shot is from season 4, episode "The Screenwriter", when the guys respond to the fire at the toy factory that also made fireworks.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy