Revealing mistake: Episode 6, while in the barn Von Weich asks Moor (played by David Troughton, son of Patrick, who plays The Doctor) to retrieve the monocle in his pocket. Then, when Von Weich continues with his plan, in his closeups we can see a vertical piece of mesh tape over the actor's right eye. (00:13:05)
Revealing mistake: When the resistance are storming the landing bay, one of them punches at least a foot to the right of a guard's face, but he still collapses.
Answer: TARDISes are generally available for properly authorised use on Gallifrey; they're not usually assigned to a particular Timelord on a long-term basis. The Doctor stole his when he left his homeworld.
Tailkinker ★