Revealing mistake: In the scene at the golf course, Mary Tyler Moore and Donald Sutherland are supposedly putting out on the 18 green. If you look carefully, you'll see more than one hole cut in the green. They're actually on the practice putting green.
Ordinary People (1980)
1 revealing mistake - chronological order
Directed by: Robert Redford
Starring: Donald Sutherland, Timothy Hutton, Judd Hirsch, M. Emmet Walsh, Mary Tyler Moore
Genres: Drama
Other mistake: At the beginning, when Conrad drives with his friends, one of them takes his head out of the window and says, "I need to Jump". The kid next to him is mouthing his lines.
Calvin "Cal" Jarrett: Well, don't admire people too much. They'll disappoint you sometimes.
Trivia: Robert Redford won an Academy Award for Best Director for this movie, which was his only competitive Oscar victory.