When Bart looks at the picture of Mrs. Krabappel for the first time, her eyes are open, but when the family see it later her eyes are closed.
During the "World Without Zinc" short that Bart it watching at the start, a guy is depicted failing to start a car because the car has no battery without zinc, yet the sound of the engine cranking over is heard. No battery = no crank.
Bart writes all of Woodrow's letters by hand. Mrs. Krabappel is his teacher, but doesn't seem to recognize his handwriting.