It's important that you don't read this question if you haven't watched episode 1 of season 6, "The Red Woman".
No-one who has been paying attention to either the show or the books will be surprised to know Melisandre's necklace
has some sort of magic properties. We know it somehow protects her from poison, and some of us suspected it somehow aided in her charms/glamour.
However, there is an inconsistency. In an earlier scene from another season, we already saw Melisandre's
naked body while she wasn't wearing or touching her precious necklace. For example, in this scene where she bathes naked while Lady Selyse is watching her. She looks young, not like a hag. Some fans have commented there is a suitably odd expression in Lady Selyse's face while she's talking to Melisandre, like she's noticing something we, the spectators, don't.
But it doesn't make sense. Whose point of view was that earlier scene? Was Meli's magic working in some other way? Was it a case of artistic license?
Why would Lady Selyse see something different than us?
Or were we deceived in that earlier scene, and only now the deception has been revealed?