Other mistake: When the Doctor pulls Zoe and Jamie back into the TARDIS, look at the scanner screen. It has the words 'Producer Peter Bryant' written on it.
Other mistake: The Master says he wrote 5,000 words a week for 25 years and Zoe says "but that's over half a million words." Yes it is, to be exact, six and a half million words. (5,000 x 52 x 25 = 6,500,000).
Continuity mistake: This story starts off where the previous story ('The Dominators') finished, but Jamie says something completely different to alert the Doctor to the approaching lava flow.
Continuity mistake: Near the end of episode three, Jamie starts to read the printout. At first, he is reading the printout the right way round. But when we next see him, he seems to be reading it upside down. Then the third time we see him, he is back to reading it correctly (right way up).
Continuity mistake: When the TARDIS breaks apart and the console sent spinning off into space, Zoe is lying on it. In close-ups she lying on her left side facing towards Jamie (who is opposite), but in the distance shots done with small models, she is lying on her right side facing away from Jamie.
Chosen answer: The Master is insane. If he can't control the universe, he'd rather see it destroyed.
Captain Defenestrator