Factual error: The Viking village has tubs of electric eels in the boathouse, which prove instrumental to the plot as the power source of the makeshift electromagnet used to relieve several of the attacking aliens of their helmets. The problem is that the Vikings live somewhere in northern Europe, and this episode takes place in the 800s. Electric eels are native to the Amazon and Orinoco rivers of South America, which at this point in history no one in Europe knew existed.
Factual error: By halfway through the episode, the plot is alternating between following Clara and Jac in London, the Doctor in the fictional Central Asian country of "Turmezistan", and Kate in New Mexico. Despite the three locations being scattered all around the world, only in London is it depicted to be nighttime at any point in the episode. Central Asia and the southwestern USA are both shown to be daytime, despite the two locations being over halfway around the world from each other, and the events all taking place at roughly the same time.
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