Continuity mistake: When John and Roy are with the possible MI, while Roy's setting up the IV bag John's on the Biophone with Rampart, and after Brackett orders the IV TKO with MS, we see that Roy's pant leg has nothing on it, but then it cuts to the closeup and suddenly there are strips of torn tape (for IV as usual) on Roy's pant leg.
Continuity mistake: When Station 51 is dispatched to the structure fire including a chemical lab, Johnny and Roy put on SCBA tanks with distinctive markings/stickers before going in, but those tanks change while inside and when they exit with the victims of chemical smoke inhalation.
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 Billy's mother waits in the hallway while Early examines Billy, the window behind her is on the same side as the ER's receiving entrance. Note that outside the window there's a blue metal shade at the top, which does not exist in any exterior shots of that side of the ER.
Continuity mistake: When Captain Robertson voices his disapproval about the paramedics, while the guys are in the kitchen eating, there are plates, cups, saucers, and utensils on the table. Then Station 51 is called to respond to a child trapped at the storm drain, and when the guys run out of the kitchen we see behind them that the table is now bare.
Continuity mistake: When Brackett asks Early and Morton to assist with the examination of the hallucinating veteran, in the shot from the hallway the doctors walk into treatment 1, its door has no window, but when it cuts to the shot from inside the room, its door has a window, which means they're all in treatment 2 or 3, not 1.
Other mistake: All the guys are sitting around the table in the day room discussing how smart kids are, and when the tones drop we hear the dispatcher say, "Squad 51 possible assault victim," yet all the guys from the engine company rush off and head to the rig in the apparatus bay, even though only Roy and John were called.
Other mistake: When Dixie and Mr. Hart are on the elevator, we see the number five lit up on the panel above the door as they head down to the first floor in the ER, where they meet Johnny and the kids, who get on the elevator to head up to the fourth floor. The problem is that the ER is only a 2-storey building.
Visible crew/equipment: When Johnny asks Dixie for advise on how to talk to fifth graders, Dixie walks out from behind the desk at the nurses' station, and as they walk up the hallway the actor's tape mark is visible on the floor, where she had been standing and where Morton is about to stand.
Visible crew/equipment: When Brackett, Early, and Morton enter treatment 1 to examine the hallucinating veteran, inside there's a young doctor standing to the left of the door. After Brackett tells Morton he wants an EEG, skull x-rays, and brain scan, when Morton heads to the phone to order the neurological workup, we can see the actor's white tape mark under the gurney, where the young doctor had been standing.
Revealing mistake: After Station 51 is dispatched to the 'plane in trouble' it becomes a plane crash rescue and recovery, and when they pull out the pilot's body, who they said died on impact, his right hand moves to clasp his left wrist, and he also bends and moves his leg.
Visible crew/equipment: When John and Roy tackle the hallucinating veteran, the outlines of the mic transmitter and wire are visible on the back of Johnny's shirt. As an aside, this scene was shot right outside Universal Studios, note the Black Tower/MCA Tower (which houses Universal's execs) when John tells the veteran that it's not a trick and to calm down.