Other mistake: After Dixie gets the call from the dental clinic about the possible MI, she and Brackett take the elevator located at the nurses' station and head up to the dental clinic on the third floor. Problem is that the ER is only a 2-storey building, so that is quite impossible.
Revealing mistake: Dixie and Brackett rush onto the elevator with a crash cart to go up to the third floor, for a possible MI in the dental clinic. The camera's POV is from the elevator, and in one continuous shot, without cutting away, we see the ER hallway through the open doors, then the doors close and in a few moments the doors open on the third floor. Even though the set has changed a bit, we can tell it's still the ER hallway by things such as the same scuff marks on the floor, and we can also catch a glimpse of the paramedic status board on the wall, where the base station room is located.