Deliberate mistake: When a patient who is given CPR is seen in the shot, the person giving the CPR does so with bent arms (the correct way to do it is with straight arms). This is deliberate, as straight-arm CPR puts in an amount of strength that frequently breaks ribs. Many films do CPR with bent arms rather than straight to avoid harming the actor.
Factual error: There are several episodes where doctors continue to defibrillate a patient even after s/he flatlines. A flatline (asystole, where there is no electrical activity in the heart) cannot be corrected by defibrillation, which stops a heart that's in an abnormal rhythm and gives it a chance to start in a normal one (the heart has already stopped when there is a flatline).
Factual error: In an episode with a severely overweight patient who is thought to have diabetes, it is discovered he instead has Chagas disease. Cameron then informs the patient that he acquired this illness from his food when, in fact, Chagas disease is a protozoan that is spread through the bite of the triatomine bug.