Delta and the Bannermen - S24-E3
Revealing mistake: The bodies lying on the ground during the initial battle are rather too obviously rubber dummies rather than real people.
Dragonfire - S24-E4
Revealing mistake: At several points in the story the 'huge heavy ice cliffs' get blown fractionally upward by people running past them, showing that they are made of very thin, very light plastic.
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 ★