Battlefield - S26-E1
Deliberate mistake: Look for the "creases" in the wall inside the TARDIS, which look as if the wall was just a sheet hung up behind the console. (In fact it actually WAS a sheet and not a "solid" wall - the TARDIS wall prop was apparently unusable due to water damage, due to poor storage between seasons. This also explains the lack of TARDIS interior scenes throughout the rest of season 26: it was not thought worth repairing the TARDIS set [or building a new one] when the show was in imminent danger of being cancelled after season 26).
Battlefield - S26-E1
Continuity mistake: When the roof falls on Ace and Shou Yuing, Ace's face gets covered in dark streaks, which mysteriously disappear soon afterwards.
Battlefield - S26-E1
Character mistake: The Brigadier asks the pilot if she speaks Czechoslovakian, even though Czech and Slovak are different languages.
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 ★