Here is an example of Etiquette not understood by us humans. To have one tail flap undone is not unlike to having the fly unzipped to a point, but happens a little more often cause its really impossible to see. However its also a fashion and a social thing as well. Some guys/girls wear it undone for a statis and cause be thought as sexy or trampy or similar to how some people wear their pants to the point of falling off. Tina doesn't realize her's is unbuttoned so Beverly fixes it so people won't get the wrong idea.