Other mistake: When Doug is drawing a picture to explain to the nurse why he is there, a man purchases a beverage from the "Hot Drinks" machine behind Doug. The drink is ready unrealistically fast and the man immediately takes a sip from an obviously empty cup. Also, if the machine actually worked he would have scalded himself, as those machines notoriously produce extremely hot beverages.
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Other mistake: When Doug and Carrie go to visit the new neighbour, Jessica, Carrie tells her that they live at (street) 8013. But in many other episodes you can see the numbers 3121 on the Heffernan's door and we learn that the Heffernan's live at 3121 Aberdeen Street in Rego Park.