Background Pony Number 17
Well, that’s more charitable than the usual fan-theory: Celestia pranking Luna by not telling her how Nightmare Moon’s legend grew over the centuries.
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Page 521 - Jumping the BlunderbussMy dearest, most faithful student Twilight,Luna did WHAT?!Sincerely,
Princess Celestia
Twilight Sparkle: So, shouting is not just your specialty, but… tradition?
Princess Luna: That and the royal “we,” yes. They are components of the ROYAL CANTERLOT VOICE.
Twilight Sparkle: Yeeeaaahh, that might be part of your problem.
Princess Luna: We do not understand.
Twilight Sparkle: Um… Wait, didn’t… Shouldn’t Celestia have briefed you on this? She didn’t give you any warning whatsoever?
Princess Luna: Oh. Well, that is… You see, while we recovered, my sister told us about the Nightmare Night festival in passing… We were so morbidly curious to see it ourselves, so excited to make our first public entrance in a millennium… We MAY have neglected to tell her that we were going.
Twilight Sparkle: …Great.