I'll Fix It - S4-E2
Continuity mistake: While Roy and Johnny are under the house with the oil eruption trying to free the trapped man, they get covered in black oil and mud even though they're wearing turnout coats. It drips down their necks and their pants are soaked with it. When they get to Rampart with the victim, Roy says that he and John should get washed up, but when they talk to Dixie they haven't just washed up, their clothes are now impossibly spotless.
Smoke Eater - S4-E16
Continuity mistake: After Dr. Early is called a miracle worker and then again tries unsuccessfully to get the apple from the machine, it cuts to Squad 51 pulling backwards into the apparatus bay, and the engine we see parked in the bay is the Crown from the first two seasons, not their current Ward LaFrance.
Continuity mistake: When Roy and John leave the ER after the baby delivery, the base station dispatches them to the motorcycle accident, and in the next shot as the squad turns the corner, there are three people in Squad 51, we see Roy, Johnny and someone seated between them, while they're en route.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy