In "The Rings of Akhaten" why do you think Clara nevr followed up on the Doctor's statement? She started to and events kind of took over at the time but there was time later when she could have. Also this was only the 2nd story of season 7 part 2 so there was a lot of time to bring it up. Do you think she just forgot? Not likely. Could this have been a throw away line? The way Doctor Who is run today that is more likely. Or is Steven Moffat like John Nathan Turner where he gets an idea, put it in the story, forgets about or changes his mind and continue on as if it never happen?