Something that's been bothering me for awhile.
Fake locket Horcrux: if it is indeed Regulus (or whoever, really) who took it, how come there was still potion in the stone bowl? Well, other than the obvious, as to perpetuate the idea that the real one's still there. Was it the same potion? (And if it was, how would Regulus have known which potion it was...) Would it have been a similar but more poisonous one, in a half-hearted attempt to kill Voldemort right then and there?