Other mistake: In the scaring simulation room, the alphabet wallpaper border at the top of the wall has the "J" written backwards.
Suggested correction: The walls of the room also have nonsensical drawings like a giraffe with two heads. The implication is that the monsters designing the room didn't entirely know what kids would actually have in rooms (like they didn't know that giraffes don't actually have two heads). So they thought that humans have the J backwards.
Whether or not the backwards J was intentional or as a joke, it would still seem unreasonable for the monsters to get it wrong when you see they got the pictures for each letter seen correct "jaguar", "kangaroo "pig", "quail", "rhino." I would at the very least call it a character mistake.
As the original correction said, it's just an example of the monsters not understanding the human world. Not a mistake.
Yet they know the alphabet? I doubt it.