This story idea centers around Starlight's complete disregard for ethics when it comes to using her magic. We've all seen it; "Every Little Thing She Does", "A Royal Problem", what she did to Big Mac in "No Second Prances", Starlight has almost absolutely no regard for any boundaries and just does whatever she wants with her magic. And you know the main reason behind this? It's Twilight dealing out absolutely zero punishments whenever she does something like this. Unintentionally, the purple pony was enabling this behavior in her student because she never had her suffer any ramifications for this irresponsible use of her power.

So here's my idea: Twilight ends up getting a message from Manehatten or even Canterlot about Starlight. Her graduated student had found herself in the slammer after she had used her magic on another pony unjustly.

Being her, she'd go off and try to get her friend out of jail. Most likely by trying to explain that Starlight can "overdo" it with her magic, but that her intentions are always from a good place. And that's when reality ensues; after all, "The road to Hell is paved with good intentions". Twilight would end up getting chewed out for, not only realizing that Starlight uses her magic in such a reprehensible manner, but sweeping it underneath the rug and making an excuse for it instead of actually doing anything about it. Even calling into account her own competence as a teacher and whether or not she made the right decision in "graduating" Starlight so soon.

https://www.youtube.com/watch?v=fH-5KM-X1SI&feature=youtu.be&t=3m2s