Other mistake: During the fire at the paint factory, just after the barrels at the back of the pickup up truck explode, in the next shot we see Marco and Chet manning a hose. In this closeup, another fireman runs up to them and he's wearing a helmet with 51 on it, but it's obvious he's not one of 51's guys, plus Roy and John are with the burn victim and Mike's at the engine. The fact that it's a closeup and we can see perfectly well that he's not actually from 51 makes this too odd.
Other mistake: After Mr. Winthrop is brought to the ER, there is a closeup of his EKG monitor as it's picking up multiple PVCs, but when Brackett asks him about the tension in his life, the monitor seen behind him is showing a flat line. This happens once again after Roy and John leave the treatment room, just before they move him to CCU.
Other mistake: After Roy and Johnny get back to the station, when Johnny takes the container of milk out of the fridge, we can see the black tape around it covering the brand name. There's another container in the fridge with red tape covering its brand name as well.
Other mistake: At the structure fire, Roy and Johnny rescue 17's Captain Harmell, as seen on his helmet after he fell through the roof, but when Truck 17 sends its snorkel to the roof, 51's Captain Stanley and Captain Watson, whose helmet also shows 17, both assist with the rescue of the other Captain of 17.
Other mistake: When Johnny and Roy are at the construction site with the injured father and son, Rampart is also on the line with two other squads - Squad 18, treating a pulmonary edema, and Squad 36, treating a bike accident. When Marco and Chet carry up the stokes basket, we hear the dispatcher saying, "Engine 21 and Squad 18, heart case...," even though Squad 18 is already tied up with the pulmonary edema.
Other mistake: When Roy and Johnny are working on the squad's engine, while Johnny's lying on the mechanic's creeper, Roy is holding a spray can with white tape wrapped around the top of the can, so the brand name would not be visible.
Other mistake: After Dr. Early fixes the squad's engine problem, while Roy and Johnny are driving away, Roy's watch reads 10:30, but when the dispatcher sends them to the leg injury he says that the time out is 11:52. Then at the scene of the football player's leg injury, Roy's watch reads 8:15 and Johnny's watch reads 9:30.
Other mistake: When Tommy is in the treatment room drinking chocolate milk, the container of milk has tape wrapped around it, so the brand name would not be visible. Later at the station, when Chet is polishing the table in the kitchen, the spray can he's using has tape wrapped around it as well, for the same reason.
Other mistake: When the dispatcher sends Squad 51 to the heart attack at the bank robbery location, the address is given as "2057 North Hillcrest, cross street Barker," but when they arrive at the corner bank, the sign reads Celis Street.
Propinquity - S3-E20
Other mistake: When Station 51 is dispatched to the traffic accident, while en route first we see Engine 51 with nobody on the back, but then we see two firemen standing at the back, and they are Johnny and Roy. This footage is from the episode "The Old Engine" in season 3, when Station 51 got the new Ward La France engine, and since the previous shift was still out with the squad, Cap told Roy and Johnny to jump on the back and join them on their first run with the new engine.
Inventions - S3-E22
Other mistake: After Johnny and Roy are seen carrying the semi-conscious worker during the chemical leak, Captain Stanley tells Mike to get the life gun, and tells Chet to secure a lifeline, then before he mentions getting the guys from the catwalk via the lifeline, we hear Captain Stanley tell the Battalion Chief (into his handie-talkie), "Battalion 14, Engine 41," even though he's Engine 51, not 41. Additionally, when Cap tells Chet about the lifeline, two firemen behind Chet are wearing helmets with no company numbers on them.
The Promise - S3-E11
Other mistake: When Johnny and Roy get back to the station, Chet tells Johnny that Paula's going to come by around 12:30 PM, then when the dispatcher sends Squad 51 on the 'woman down' run he says "Time out 12:22." When the guys get back to the station again, Paula tells Johnny that she arrived around noon and that Engine 51 left on a run shortly after.
Other mistake: When Battalion 5 orders Copter 10 and Copter 14 to make an air drop on Jawbone Canyon, it cuts to a shot of Copter 14 taking off. This is the same shot used in S3, E6, "Snakebite", when 85's guys tank Copter 14 in order to make a water drop where the teenagers' car is on fire down below.
Other mistake: The Battalion chief assigns tanker 76 to assist engines 51 and 60 with the rescue at the canyon. However, it is tanker 44 that arrives.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy