When Mr. Mackey is singing "It's Easy, Mmmkay," the writing on the two chalkboards repeatedly change very quickly, even though there was obviously not enough time for anyone to have erased them and write over them.
Usually when things in a movie background change from one shot to the next, it's an error. But this was very deliberate, even if it is impossible. It's no more of a continuity "error" than Kenny being alive in an episode despite having died in the preceding episode.
It's as if Parker and Stone just want to say "Fuck continuity!" Good for them. As long as the TV show and movies stay funny, then fuck continuity.
--- Fuck is the worst word that you can say. Fuck is the worst word that you can say. So we shouldn't say fuck, No we shouldn't say fuck, Fuck, no!