Factual error: In the scene where Lisa's great great grandmother, and the slave Virgil escape to Canada, they enter Canada across a suspension bridge, which has, on the Canadian side, the Canadian Maple Leaf flag. In 1860, Canada was not yet even a country, and the flag was the Union Jack and the Canadian Ensign. The Maple Leaf was not adopted as the Canadian flag until February 15, 1965.
Continuity mistake: Ms. Krabapple takes the cellular devices from the children in her class and puts them in a Laramie Cigarette box. She dumps the box into a desk drawer filled with frogs. A few seconds later there is a shot of the class from her point of view with all of their cellular devices on their desks.
Continuity mistake: When Homer is showing Carl and Lenny the large bottle of wine he has found in the wine cellar, you can see there is nothing on the shield on the label, but when see Homer holding the bottle a few seconds later, the shield has now two lines on it.
Continuity mistake: When we see Homer in the audience at the conference at the beginning of the episode, he is sitting between a lady with grey hair and a man with black hair. But when we see a wide view of the audience a little later on, he is now sitting between a man with brown hair and a bald-headed man.