When your child consistently selects characters from mythology, the arts, or literature for the subject of dress up or Halloween. Yes, the costume-which-must-be-explained-in-detail. {sigh}
This year it's The Snow Maiden. She looks amazing... but already knows that it's mostly going to be lost on everyone else. She might be anything from Elsa to Marylin Monroe, and the guesses are likely to be that far afield, if past experience is anything to go by.
Back in the day at Harry Potter parties, it was Fleur... (which, once explained, got looks of downright AWE because it was that good a costume-- DD liked it for the "No Fairy Princess" quote from the books, however).
This one is another one that good, but even more obscure. DD has a reputation of being a "cold fish" among her more volatile college peers, particularly in theater. Perfect.
But obscure. I figured this is a place filled with other parents who can identify-- and just noting that they never really grow out of it!