Factual error: When Roy is electrocuted and falls from the roof, after Karen uses the defibrillator paddles on Roy, she lifts both paddles, looks at the EKG monitor and says "He's converted." How exactly could Karen have known that he's converted? It's impossible for the EKG monitor to show anything at all. Either the defib paddles have to be in contact with Roy's body for the "quick-look" to get a reading, which they weren't, or the ECG electrode discs have to be on Roy's chest connecting him to the EKG monitor, and they weren't. As an aside, just watching Marco having problems attaching the air mask, and quickly glancing up towards the camera frustrated, then giving up is priceless.
Visible crew/equipment: While rescuing the invalid, when Roy climbs out of the second floor window he suffers an electrical shock, and when he falls in the closeup we can see the crash mat that he lands on when part of it flips up from the force of his weight. Also note that all the shrubs below the window have vanished.
Other mistake: After Chet's practical joke on Karen, she's quite ticked off, and Roy and Johnny are a bit uncomfortable, and then when Station 51 is dispatched to the unknown type rescue they all head to the apparatus bay. In the next shot when Karen, Roy, and Johnny get into the squad just watch all the actors' faces. First Johnny cracks up, then Roy and Karen, who even put their heads down to hide their laughter, particularly Roy who ducks his head behind the steering wheel. Fun to watch.
Visible crew/equipment: When Roy tells Karen that she'll know when she can do it on her own when she'll have to, the reflection of set lighting is visible on the squad's chrome spotlight. Additionally, when Station 51 is dispatched to the structure fire the rear bay door closed, but when the rigs roll out the rear bay door is open.
Other mistake: After Station 51 arrives at the structure fire, Captain Stanley tells Mike to get water to their lines (charging the two lines) and then mule the supply line back to the hydrant. When Marco and Chet get the two lines, they head to the house with Captain Stanley and start hosing the fire down with charged lines. A few shots later, we see Mike going to the back of Engine 51, and the two lines that Chet, Marco, and Cap have been using are flat and not even charged yet.
Visible crew/equipment: After Karen, the new trainee, goes to the waiting room to speak to the elderly patient's wife, when Dixie walks over to Roy and Johnny to ask how she's doing, the outlines of the mic transmitter and wire are visible on the back of Dixie's white uniform.
Continuity mistake: When Dixie is having a heart to heart talk with Karen, in the shot facing Dixie she reaches out with her left hand to take the napkin from Karen, but in the closeup it's her right hand that takes the napkin. Additionally, the napkin she takes from Karen is perfectly folded, but it was wrinkled and messy when Karen held it.
Visible crew/equipment: When Roy, Johnny, and Karen are waiting outside the treatment room with the elderly patient, with Brackett and then with Dix, the outlines of the lavalier mic transmitters are visible on the left sides of Roy's and Johnny's shirts, over their belts.