Factual error: A scene near the beginning of the movie when Forest Whitaker and Oprah are both speaking on different phones. The phone that Forest is using has a modular plug. The scene takes place during the Kennedy administration (early 60's). The modular phone plug was not patented and put into use until 1975.
Other mistake: When Cecil and Gloria are having their birthday celebration, before they are informed of Charlie's death, Cecil goes to the refrigerator to open a bottle of champagne. We hear the distinct pop of a cork from a bottle of sparkling wine, so we know he chose that bottle, but when he enters the living room a few seconds later, he is holding a glass of non-sparkling wine (no bubbles are visible).