You know, they may have played up all the clues in the book:
Dontos gives her the necklace, which she wears to the wedding.
The Queen of Thorns messes with her hair (does she touch anywhere around the necklace?).
Was the goblet close enough for Olanna to have dropped it in there? Margaery did carry it off in that direction.
I was too busy gleefully waiting for Joffrey to die to pay too close attention.