My two biggest pet peeves that we teach children without analysis is (a) that polar bears and penguins co-exist when they actually live at opposite poles, and (b) pictures like http://www.ferglib.org/ferg/images/stars_moon.jpg that draw stars inside the arc of a crescent moon as if the moon isn't always a solid sphere. Oh, and maybe (c) people using the "generic" male pronoun to refer to cows and hens.