Continuity mistake: K-9 looks slightly different in the scene in which he's spinning inside the TARDIS: notice the pattern on his collar.
Continuity mistake: After the Doctor helps Romana over the cliff, the stock footage of a sun setting over a calm sea does not come close to matching the total darkness and stormy seas in previous shots.
Continuity mistake: During Kroll's attack on the refinery, several pieces of the refinery get broken off and then reappear in subsequent shots.
The Armageddon Factor - S16-E6
Continuity mistake: In episode 2, just after K-9 begins blasting the door, look behind Romana and notice that you can see the TARDIS - which supposedly left in the previous scene.
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 ★