Continuity mistake: When Anna is holding the "virtual pet" after realising it has died, in the shots from the front she is holding it in both hands, but from the back it's only in her right hand. It's tricky to see because there is a pile of papers in the way, but her left hand is folded in front of her in the shots from behind.
Continuity mistake: Peter, Elizabeth, and Romano are with the little boy who has organ reversal and as Romano says "genetic breakthroughs are in the wings", Peter puts on one glove. As Lizzie asks him if something is wrong, he is putting on the glove again. As they continue to talk, he begins to examine Rodney, but when the shot changes, he is still putting on his gloves.
Continuity mistake: In season 2, Doug mentioned that he hadn't seen or spoken to his father in 22 years, since he was twelve. But when he's going through his father's things at the motel, he finds pictures of himself from high school. Plus, he mentions his father teaching him how to drive when he was about 13 or 14.
Continuity mistake: Romano is standing outside an ER room watching through the glass. To his left, down the hall, we can see a woman pushing someone in a wheelchair. But when the shot goes to him entering the room, there's an extra behind him on the left pushing a large cart of supplies - there was nowhere for him to come from.
Continuity mistake: Doug goes into the ER room to respond to the boy, who's saying "go away." His mother has her arm around his head and is telling the drunk with his teeth out to go away. Then the shot changes and we see the boy's head, but no mother. A quick shot change and there she is again, cradling his head.
Continuity mistake: Near the beginning of the show, Doug is examining baby Josh and puts his stethoscope in his ears. Kerry says, "Your actions are totally irresponsible", and the shot goes to Doug. There is no sign of the stethoscope anywhere. The next shot, from behind him, shows it's around his neck. He then takes it off and the next shot should show it in his hands - but it's disappeared again.
Continuity mistake: Carter rushes over to a Coke dispenser where there are four plastic jugs filled with ice and we see him dump all four of them into a plastic tub. The camera goes to Anna, who says "Carter, Carter" and then we see him dump the last two jugs again. When Carter reaches the patient, there is considerably more ice in the tub than he had to begin with.
Continuity mistake: In the OR, Peter is trying to save Dr. Swanson's life. Lizzie is standing in the corner watching. She has her right hand up to her chin, resting the elbow on her left hand, which is at her waist. Subsequent shots show the opposite - her left hand on her right.
Continuity mistake: Within the space of what would appear to be more than 5 minutes in the ER, considering the continuous flow of action (starting with Kerry offering to get Pablo a taxi and ending with Dr. West examining the patient Lenny) the clock goes from 10:30 to 6:20. Shortly after that Dr. West pronounces Lenny dead, saying 13:38 (or 1:38).
Continuity mistake: When Carol goes to get Doug to see the sickle-cell patient, the clock reads 3:50. The next few scenes include Rachel arriving at the hospital and being taken to the dentist and for a dual makeovers by Cynthia. When Mark is walking by the same clock with Herb the lawyer, it reads 2:30. He then says goodbye to Herb and his ex-wife arrives to pick up Rachel, who is now back in the ER with Cynthia. The clock times do not reflect the sequence of scenes.
Other mistake: When Doug and the nurses are watching the TV about a motorcycle accident, Kerry comes along and calls him Mark. This could be a character error, but as Doug responds 'yeah', and they talk, it indeed becomes a production error. In real life, the person would have corrected her.
Continuity mistake: When Peter comes in to talk to Rodney's father Isaac, the clock on the wall says 11:40 - which it had also read a few scenes earlier when Rodney came out of surgery. After Peter checks the chart and starts to talk to Isaac, the clock has somehow changed to 3:30.