Other mistake: After Station 51 is dispatched to the structure fire, just as Roy and Johnny start heading upstairs we see footage from 4x3, "Gossip," when Johnny brings the hose to the stairs at the chemical fire, but the line doesn't reach so he leaves the hose at the bottom step, and they continue up.
Visible crew/equipment: During the structure fire, after Roy and John climb out onto the ledge and the fire flashes in the exterior shots, the next interior shots we see shows stacked boxes falling as the fire flashes through the doors. The interior shots are from episode 3x7, "Promotion", during the structure fire, after the security guard is taken down the ladder. We can even see a crew member's hand pushing a lower box which causes them all to fall.
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.
Visible crew/equipment: After Johnny and Roy save the man at Vic's party, when they are leaving, the reflections of crew members and production equipment are visible on the orange surface of the ambulance as it drives away.
Visible crew/equipment: At Vic's party, when Roy and Johnny are walking up the steps of the house (just before seeing Monique), the reflection of the cameraman holding the handheld camera is visible in the door's glass, when he steps backwards as the guys walk in.
Continuity mistake: While Johnny, Roy, and Vic try to outrun Charlene inside Stage 3, after heading down the ladder, when Roy opens the door to find Charlene standing there, Johnny's holding the red extinguisher in his right hand and the nozzle in his left hand, but next shot they've switched hands.
Continuity mistake: When Roy and John are talking to Vic at his party, his manager's cigar changes length repeatedly, from short to long and back again, depending on the camera angle.
Visible crew/equipment: When Roy and Johnny are with Vic (Adam West) trying to outrun the bear, the outlines of the Lav mic transmitter and its cable are visible on the back of Johnny's shirt.
Revealing mistake: When Station 51 is dispatched to the bomb threat, just as they roll up on scene, in this specific POV the corner edges of the screen are blacked out due to the lens that was used, but when it cuts to other angles it's gone, then it reappears with that POV. We see it three times.
Continuity mistake: When Dixie tells Roy and John that Vic wants to see them the guys walk into treatment 2, but in the shot from inside the room we can see that they're actually in treatment 3, on the opposite side of the hall.
Visible crew/equipment: When Roy, John, and Vic are inside stage 3 trying to outrun Charlene, Roy starts climbing down the first ladder, and we can see equipment moving right beside Roy, then when Roy climbs back up and they find the second ladder, more movement can be seen at the bottom of the screen.
Continuity mistake: After Roy and Vic run out of Stage 3, when Johnny backs up and bends down, we can see Charlene, the bear, on all fours, then in the next shot the bear is walking on her hind legs toward the door, but in the next shot from behind Johnny the bear is still on all fours and further back again.
Visible crew/equipment: After Station 51 is dispatched to the bomb threat, when the squad and engine pull into the alley, just as Roy stops the squad, the reflection of the cameraman/camera are visible in the spotlight's chrome surface.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy