Camera Bug - S4-E10
Other mistake: When Engine 51 and Squad 51 first roll in during the school fire, we see the burning building has a flat roof. After Johnny and Roy have rescued the unconscious boy from the classroom, there is a closeup of two firemen standing on a slanted, tiled roof with a hose, which is from episode 3x2, "The Old Engine", just before 17's Captain Harmell falls through the warehouse roof.
Camera Bug - S4-E10
Other mistake: While Roy and John are at 68's, Roy goes to the stove to see what Bob was cooking when he started having pain, and the closeup footage we see of the pot of bubbling chilli is the same as some of the footage of Chet's chilli in 4x1, "The Screenwriter," when 'Miss October' stops by the station.
Camera Bug - S4-E10
Other mistake: At the end of the episode, when Johnny's telling the other guys that his photograph was bought, the box of cereal and carton of milk that are on the table have tape affixed to them, to deliberately cover their brand names.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy