How did Melisandre stay young the first time her necklace came off?

There are two theories about how Melisandre stayed young in the scene without the necklace. The first theory is that it was simply an error in the show. The second theory is that Melisandre was deliberately not wearing the ruby choker in the bathtub and that she really was old in that scene. The only person who could see her in that scene though, was Selyse, who was a devout follower.

Melisandre claims to have powers that allow people to see her as she really is, so maybe Selyse just saw Melisandre as a young woman because that is how Selyse wanted to see her.