Revealing mistake: After the snakebite, when the guys carry Johnny and place him at the back of Engine 51, the chrome spotlights are shiny and reflective, but in the closeups the spotlight near Marco has been partially sprayed with a dulling spray, to prevent the camera's reflection.
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.
Visible crew/equipment: When Roy and Johnny are in the basket with the man who was electrocuted, after they finally get the victim on the ground and surround him, the shadows of the cameraman/camera are visible mainly on Roy and Captain Stanley.
Smoke Eater - S4-E16
Character mistake: When having dinner Chet asks Marco to pass the salt. Marco hands him a bottle of ketchup instead.
Answer: It is standard practice to wet down driveways so that they stand out in a long shot.
stiiggy