In the first season installment "Divorce Isn't Everything", Mary mentions that she can't speak French but can speak Spanish. Later in the series, while at a Mexican restaurant, she indicates that she can't read the menu because she took French in college.
Exteriors of Mary's residence show that the apartment with the balcony (which would be Mary's, based on the interior shots of her apartment) is on the top floor, yet Rhoda's apartment is upstairs from Mary's.
On the newsroom wall there are various world clocks like you would see in a typical newsroom of that era. In the center is the Minneapolis/St. Paul clock, and to its left is a Chicago clock. Minneapolis and Chicago are both in the central time zone, so there is no need to have two different clocks. Furthermore it can be seen that the Chicago clock is set roughly an hour ahead.
Exteriors of Mary's residence show that the apartment with the balcony (which would be Mary's, based on the interior shots of her apartment) is on the top floor, yet Rhoda's apartment is upstairs from Mary's. However, Rhoda's apartment is located in the attic of the building.
Two vertical painted piano hinges may be seen to the right of the door in Lou's office between it and the adjacent wall. In addition, his desk is mounted on castors, both used to facilitate breaking down the set.
The exterior establishing shot of the building where WJM is located, usually zooms closely to a window, but the newsroom has no windows, except Lou Grant's office- which opens onto a brick wall. There are no nearby buildings with brick walls.
Throughout the series there are several continuity errors involving the clocks mounted on the wall behind Mary and Murray's desks. However, looking closely, you can see that they are not even running, as the second hands remain stationary.