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.
Audio problem: While Roy and John are with the asthmatic boy, Roy's on the Biophone with Rampart, and we hear Early say, "Administer 0.3 milligrams epinephrin IM," but his mouth is not in sync with the last part, and it looks like he actually said,.". Epinephrin IV."
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: 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.