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.
Deliberate mistake: When the two lawyers were wheeled in by ambulance riding on the same gurney entered the ER the male had a bandage on his foot because they had been walking on hot coals. The hospital staff smelled smoke coming from the bandage on the male lawyers foot. It burst into flames and Abby used a Dry Chemical Fire Extinguisher. Something other than powder came out because the dry chemical powder would have made a large cloud of actor choking dust all over the set. The use of a C02 would have been a better choice. You would still get the fire fighting effect with out the mess.
Answer: A man named Paul Sobriki comes into the ER in the season 6 episode 13 " All in the Family." He is complaining of headaches, Carter and Lucy decide to rule out meningitis and do a lumber puncture. Paul struggles during the procedure and believes that both doctors are stabbing him. Lucy, who is interested in Psych thinks something is wrong with their patient mentally, but Carter is dismissive of her. She gets cross with his attitude and she calls the psych department anyway to have a look at him. Later on and near the closing 20 minutes or so of the episode, all the staff start playing loud music and get a cake which appears to be blue lol, together to party as it's Valentines Day, Carter returns from somewhere and asks where Lucy is. He goes to check one of the rooms for her and Paul Sobriki comes up behind him in the dark and stabs him twice in the lower back, he collapses and as he falls unconscious he sees Lucy on the ground also. In the next episode, Be Still My Heart (ep 14), Dr Weaver discovers them and all of the staff work to try and save them, whilst Carter survives. Lucy's wounds are fatal and suffers a pulmonary embolism as Corday and Romano try to save her. Carter is grief stricken and feels guilty about the way he treated her, he becomes addicted to painkillers and Benton, Greene all help him and book him into rehab.