Other mistake: In the scene where Yakko, Wakko and Dot are sitting at their desks repeating Miss Flamiel's every word, Wakko is drawn without legs (only feet) during one shot.
Beavis and Butt-head vs. the Vending Machine - S4-E28
Other mistake: When Butt-head is purchasing the pork rinds from the vending machine, he notes that the selection for them is F2. When he is pressing the buttons to dispense them, he presses the F button twice, but never the 2 button. Somehow, the machine still dispenses the correct item.