Continuity mistake: When Johnny and Roy are heading to the parade in The Old Engine, and get sidetracked dealing with a fire rescue, they are wearing the old-style shirts with a ten-button, square bottom bib front, with five buttons on each side. When Captain Stanley laughs at them on the roof, their shirt style differs in the close-up, it's now a nine-button, pointy bottom bib, with four buttons on each side and one button at the bottom point.
Visible crew/equipment: When Johnny and Roy are on their way to the parade in The Old Engine, in close-ups facing them, the reflection of a crew member's hand holding camera equipment is visible on the hood of the engine.
Other mistake: After the apartment house fire, the restored old engine is trashed by a hail of bricks. Later, when Roy and Johnny are talking about who won the first prize, The Old Engine in the back looks nothing like it did when it was wrecked on the street. Also, the shot of The Old Engine that's shown here, is actually the same footage shown in season 3, "The Old Engine", with the lighting dimmed to signify nighttime, when at the end of the episode Roy and Johnny decide to keep it. Note the exact same broken pieces of windshield, tires, hubcaps, dirt, shadows, everything.
Visible crew/equipment: When Johnny, Roy, and Chet are removing the woman from her car, the bulging outlines of the mic transmitters and wires are visible on the back, left side of Johnny's shirt, and back, right side of Roy's shirt.
Continuity mistake: While on the way to the parade in The Old Engine, when Johnny and Roy deal with the fire rescue and reach the roof, Johnny has a rope bundle swung across from his left shoulder to his right hip. However, in the next shot as they run across the roof, the bundle is now swung from his right shoulder to left hip, and the rope is also bundled differently.
Continuity mistake: When Roy and Johnny are driving to the parade in the fixed up old engine, they repeatedly switch between the road's right lane and left lane, depending on the camera angle.
Other mistake: When Johnny and Roy leave for the parade, they turn to their left when leaving the fire station. When Station 51 is toned out to the structure fire, where John and Roy are, they turn to their right when leaving the station.
Continuity mistake: When Roy and John are at the nurses' station talking to Brackett and Dixie about The Old Engine, at the bottom right corner of the screen we can see the patient's metal clipboard chart being held, before Early even walks over to the nurses' station holding a chart. Then when Early does walk over he stands to Brackett's right, but in shots facing Dix and Johnny we still see that chart being held, where Roy is supposedly standing, not Early. Additionally, the charts hanging on the chart holder change/vanish between shots.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy