Inspired by the cover art (by glittering-pony).
Twilight hated when her teacher did it, but couldn't talk to anypony about it. After all, who would believe her if she said that Princess Celestia – always so refined, graceful and regal – could be even worse than Twily's own mother when it came to grooming? Nopony, that's who.
Except... Cadance might actually surprise her on that one.