Revealing mistake: Whenever you glimpse the inside of the TARDIS from outside, a frontal view reveals a picture of what's inside which has been placed at the back of the box. Sadly, it isn't on the sides. This means that, from the right angle, you can easily tell that it is just a box.
Continuity mistake: Throughout series 5 and series 6, The Doctor's hair style keeps changing sides.
Chosen answer: The Master knows that deep down, he deserves death for the crimes that he's committed throughout his life, and since he regards The Doctor as his arch-foe, he expects it to be at his hands. The fact that The Doctor is still willing to forgive him for all of his crimes hurts him more deeply than death would.
Captain Defenestrator