Didn't the story get the formation of the Earth correct to within a hundred million years or so? I'm pretty sure the Doctor said it was 4.6 billion years ago, which is pretty close to the accepted age of 4.55 billion years. I don't know where this idea about hundreds of trillions of years came from.

Also, I thought "Dark Times" was supposed to be understood literally rather than metaphorically: the time before the first stars began to burn. But I've only watched the episode once and I was partially distracted throughout.

Also, I think "Arachnos" was first draft. By the second draft they had become "Racnoss", clearly a substantial step forward in sophistication.