Continuity mistake: When the family goes to throw Carla through the window, watch the window they throw her towards. The window is made up of at least three sections of glass, each no more than 2-3 feet wide with metal posts between sections. When the shot changes and Carla (well, the dummy of Carla) hits, the window is now only two sections, each about 5 feet wide. The easiest way to tell is watch the metal post farthest to the right. When the shot changes to outside, that post, which should now be farthest to the left, has vanished.
Deliberate mistake: In one of the later episodes of season 2 Turk is seen being bored in surgery and checking the patient's wristwatch to see how long they have been operating. Patients are not allowed to wear jewelry or watches during surgery as there is a VERY high risk of infection and it might kill them.
Continuity mistake: During the scene where the class is being warned about the dangers of being a "wait and see" type of doctor, JD imagines himself as being the only person in the auditorium. In the first shot of him alone, there is a bookbag (or something similar) sitting on a seat about three seats to JD's left. In the next shot of him, it is gone, and in the third and final shot of him alone, it is back again.
Continuity mistake: When J.D lends his nasal spray to Doug in the final scene Doug returns the spray to J.D after putting the cap back on. In the very next shot as J.D says "Don't bogart my nasal spray", J.D uses the spray without putting on the cap, and you can see the cap placed on Doug's tray.
Character mistake: Throughout the series, JD and Turk are shown to be major sitcom geeks. At one point in the episode, JD says to Turk he Marcia Brady'd his ass, and explains what it means. Turk responds along the lines of "Yea, I know. 'Marcia Gets Creamed' Season 5 episode 3. Don't ever question me about The Bunch!" 'Marcia Gets Creamed' is season 5 episode 10. Episode 3 is 'The Elopement'. Not a mistake a couple of geeks would make.
Continuity mistake: When Laverne does a slam dunk, you can see that the ramp leading to the hospital main entrance has no-one standing on it or in front of it. But before and after the slam dunk you see see two doctors with a basketball on the ramp, and Franklin standing remarkably still, with a camera, in front of it.
Character mistake: JD is having problems with someone dressed in a gorilla suit, who is slightly shorter than he is. JD sees the janitor, who is considerably taller than he is, eating a banana and accuses him of being the gorilla. If the janitor were as tall as JD, maybe, but you would not confuse someone shorter than you with someone taller, regardless of costume.
Continuity mistake: In the scene where Turk, JD and Dr Cox are talking about women, and how many they've slept with, take a look at the items on the table in the background that Dr Cox was previously sitting on, they appear and vanish. Mainly the cup and napkin dispenser appears and disappears in between shots.