I loved this episode so much. We got to see the interactions between the two sisters. This make both of them more relatable, developed characters. I loved this episode. Except for one thing - Luna's cutie mark.
It has been shown in the episode Twilight's Kingdom that Luna's dark spot on her flank is a birth mark, not part of her cutie mark. When she gave her magic to Twilight, her moon cutie mark dissapeared and the splotch stayed.
Before this, the fandom thought that spot had been part of her cutie mark. This episode showed every pony that it wasn't. This was fine. My issue is that this episode, they did this.
Apperently Luna's birth mark is on Celestial, as well as her birth mark. I understand this was just a mistake from the episode writers, but this upset me so much I had to come up with a head cannon to fix it. So...
Starlight's spell, as she said, was a quick spell that she did without thinking it through. No thought, just action. So first, to understand Starlight's spell, let's go over what it did.
-Switched their cutie marks
-Switched their magical abilities and magic color
Yeah. That's really all it did. This was obviously an advanced, but very simple spell.
The spell that gave Twilight Luna's magic, on the other hoof, was a very thought out spell, with Luna herself preforming her own part of giving her magic. Luna, knowing her own cutie mark, gave what she knew was the true part of the mark.
Basically, I think what might have happened was Starlight, assuming Luna's cutie mark was a moon with a night sky, just as a fandom did before Tiwlight's Kingdom, switched the whole thing in this episode. Luna, knowing herself, gave her true mark.
To sum it up (because this is getting really long), it's what the pony who's preforming the spell thinks. It's the thought that counts.
But this is just my very long theory. Tag me yours!!!