It's not a mistake, it's a decision. In Toy Story, Buzz clips through the Hot Wheels track by necessity. They cut away for the one second he does so, but the animators were aware it happens and chose for it to happen anyway. Similarly, the reason Elsa is viewed from that perspective is to hide the clipping while allowing the hair to follow that artistic track (she actually turns away so this can happen, then turns back). The strength of animation is that it allows art to triumph over physics. You don't have to play by the rules. The traditional Disney eyes, for example, aren't consistent between 3/4 and frontal view. They don't care. It's not what they're trying to do.
