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.