Continuity mistake: In episode three, the readout strip dangling from K-9's mouth shortens between when he shoots the infected crewman and shoot Leela.
Continuity mistake: Lowe's "frozen" makeup changes between when he emerges from the freezer and when Leela brings him a cup of coffee.
Continuity mistake: The countdown clock in Marius' lab speeds up and slows down as needed for the plot.
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 ★