Right-clicking any sign, even if there's no click event assigned to it, causes the player's arm to move. This, despite a minor issue, is inconsistent (a right-click animation is always a sign that an action was performed), and may lead to confusion in certain cases, as there's no action being performed for the animation to display (such as in adventure maps as, by default, there's nothing to indicate whether or not a sign is clickable, being the mapmaker's job to do something to distinguish them from ordinary signs).