Plot hole: There are always family members right in the room while the doctors are literally cutting into the patients. The family is always creating a fuss or whatever, until someone asks for them to escorted out. A real hospital would never let family members into the room while they are cracking open their loved one's chest.
Other mistake: Season 12, Episode "Blame it on the Rain": At the beginning of the episode, Abby is almost run over by a man driving an SUV. They exchange words and then Abby throws a supposedly full cup of coffee at him. The cup of coffee bounces off the seat of the man's car and it is obvious that the cup is empty.
Factual error: Episode Title: The Show Must Go On Season Number: 11 Episode Number 245 (22 of that season) When the woman is giving birth, they show the baby just after they finally got her out. The baby is clean, no blood and other signs of a recent birth are visible. This is especially unlikely since the doctors had to "cut her open" to liberate the baby from the birth channel.
Plot hole: During Season 5, the search for an ER Chief takes nearly a year and two nationwide searches, but is ultimately unsuccessful. At the beginning of Season 6, Romano is chosen as Chief of *Staff* in the blink of an eye, and Kerry is appointed as ER Chief just as quickly. Then, in Season 9, Kerry and Romano's positions are switched as though such things happen everyday. There's no way decisions of this magnitude would be made in such a brief amount of time.
Continuity mistake: In "Graduation Day", towards the end when Abby and her mother are in the apartment talking about her leaving, Abby is holding the baby and when the camera is facing her the baby is up on her shoulder. But when the camera is to her back facing her mother the baby is down. It continues like that through the whole scene.
Deliberate mistake: The show regularly shows characters by the Chicago River and either on or near Michigan Avenue, when they are getting lunch or walking to work. Both U of C hospital (where the hospital exteriors are filmed) and Cook Country hospital (where the characters are supposed to work) are about 5 miles from this point. This would be an extremely long distance for them to walk on a routine basis. I'm sure this is done just because Michigan Ave and the river are two of the most scenic parts of Chicago.
Factual error: Featured characters in the show always seem to be wearing brand new winter clothing - their jackets, hats, gloves, etc. always look pristine, sparkling clean, and just-purchased. Anyone who lives in a winter climate knows that winter clothes age very quickly - the pristine look last about an hour. Extras in the show, probably providing their own clothes, wear much more lived-in normal outdoor clothing.
Factual error: This is in the Episode titled "Forgive and Forget" Also called the Tank episode. When the psychotic vet steals an M-60 series tank and drives through Chicago to the hospital, there are several problems with the tank actions. If the vet had combat locked the hatches, the police would not have been able to open the loaders hatch to shoot him, but if it had not been locked, all the bouncing around as he rolled over cars would have bounced it open to lock in the open posistion. If it had been open and not locked open, it would have been visibly open(8-10 inches) but it was not during all the views on the way to the hospital. During the trip to the hospital, the main gun is in the travel lock over the rear deck, the travel lock cannot be released from inside the vehicle, and if he had ever stopped and gotten out to release it, the police would have shot him, and yet when he gets to the hospital, he can traverse the turret. And while we are talking about the turret traversing, that cannot be done from the drivers compartment, it must be done from either the commander's position or the gunner's position. When the police open the loaders hatch, they shoot straight down into the driver's compartment in the hull, but the vet would have had to be in the right side of the turret, in either the TC or Gunner seat to be traversing the turret.
Continuity mistake: Season 11: Episode 5: An interns guide to the galaxy: Near the end, when Ray asks if he can go home, we see Sam writing her number of discharged patients on the board in a close up. The camera then cuts back to the whole group and the size and style of Sam's writing changes. Most noticeable that the circle now stretches over the line between Ray and Abby's count whereas before it was in the middle of Abby's column. (00:42:50)