Revealing mistake: When the crack in the wall breaks open, the wall wobbles rather obviously.
Visible crew/equipment: As the Doctor is watching Will emerge from the priest-hole, look over his shoulder at the far corner. Someone or something appears to be moving in the shadows there.
Other mistake: What happens to Tegan's bag when the plague victim transforms into the cavalier? It doesn't appear to be left behind, even when the cavalier later vanishes.
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 ★