There's a lot of unpleasant surprises that can be found in photo albums. A third pony in what you thought was a private moment, inexpertly applied bodypaint, or just evidence of that time that your mother dressed you up like a burrito.
Twilight Sparkle was expecting to come out of Family Night with a few fresh blushes on her cheeks. What she wasn't expecting was a mysterious pair of wings in her baby pictures.
Hopefully Celestia will have some answers.
Many thanks to Present Perfect for supplying the brilliant concept, and to Evehly for the beautiful artwork.