Okay, this has to be the most obscure callback the animators have ever done. This particular scene is a flashback to when Twilight Sparkle was still living in Canterlot, at the very beginning of Season 1, Episode 1. Note the magic aura around her horn and the book she’s levitating, as well as the aura of the unicorn in the background. They’re both the same pale color.
 
In Season 1, unicorns’ magic auras were all colorless; the animators didn’t give them unique colors until Season 2. Note that this is not recycled footage. The animators deliberately changed Twilight’s aura to match the way it was when this scene would’ve occurred.