Continuity mistake: The rhyme the Doctor recites in episode 3 is different from the original rhyme heard in episode 1. (Apparently, the Doctor [William Hartnell] forgot his lines in episode 3, when he came to the point of reciting the rhyme. So he just improvised what he thought they were, rather than what had been scripted).
Continuity mistake: The dialogue between Cutler and the technician at that beginning of episode 3 is slightly from that which closes episode 2.
Continuity mistake: In Episode 1, the TARDIS crew pass out from the change in pressure as they descend into the Earth. However, at the end of episode four, they climb out on foot and suffer no ill effects.
The Evil of the Daleks - S4-E9
Continuity mistake: (Animated version). Waterfield puts down the photo of the Doctor on his desk but the next time we see his desk the photo is no longer there and then later it's back on the desk again, without Waterfield having moved it.
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 ★