User blog comment:B j/Prologue to Chapter 282: Ten Keys and Two Keys Review/@comment-4999650-20120511200856/@comment-79.85.46.28-20120511202529

I thought about that too, when I saw it, but it would be weird to mix the canon and the filler stories...

Oh, well, after thinking about it, Yukino saying "you might have noticed this already", and mentionning "the Zentopia incident", it's as if she's talking about something that happened recently, something Lucy knows of. Maybe there'll be a part in the Starry sky arc where they'll tell the story of that incident, or maybe the incident will take place during the arc, and they'll say something like "from that time on, there weren't any Celestial spirit mages in Earthland, but Lucy, and another one", without mentionning Yukino's name.