Continuity mistake: In an episode fairly late in the life of the show, Lou Grant says that when Mary came to interview for a job at the station, she bumped into the desk in his office and said, "Excuse me" to the inanimate object. However, we saw that interview in the very first episode, and no such thing happened.
Continuity mistake: In the first episode, Mary gets the job as associate producer. Later in the series, Lou tells Mary that she got the job due to the awkward way she tried to cover a run in her nylon. At the job interview, Mary isn't even wearing pantyhose, she is wearing knee socks. You do not see her trying to cover up a run in her socks.
Continuity mistake: This episode and scene has been referenced already but contains another error. In the scene where Lou is recalling the first time he met Mary, he claims Mary had asked for a job as an assistant producer when in fact he gave her the job because it paid less than the secretary job she wanted.