Other mistake: After Chet's practical joke on Karen, she's quite ticked off, and Roy and Johnny are a bit uncomfortable, and then when Station 51 is dispatched to the unknown type rescue they all head to the apparatus bay. In the next shot when Karen, Roy, and Johnny get into the squad just watch all the actors' faces. First Johnny cracks up, then Roy and Karen, who even put their heads down to hide their laughter, particularly Roy who ducks his head behind the steering wheel. Fun to watch.
Other mistake: After Station 51 arrives at the structure fire, Captain Stanley tells Mike to get water to their lines (charging the two lines) and then mule the supply line back to the hydrant. When Marco and Chet get the two lines, they head to the house with Captain Stanley and start hosing the fire down with charged lines. A few shots later, we see Mike going to the back of Engine 51, and the two lines that Chet, Marco, and Cap have been using are flat and not even charged yet.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy