Harry Potter and the Philosopher's Stone/Harry Potter and the Sorcerer's Stone - Snakes Don't Blink!
This slip-up can be found in the beginning of the movie, where the whole family goes to the zoo for Dudley's birthday. It takes place in the reptile room, while Harry is talking to the snake. Then, VERY specifically, the snake blinks. BUT... snakes do not have eye-lids, therefore, the cannot blink! They lick their eyes!