The Pyramid at the End of the World - S10-E8
Continuity mistake: While the Doctor, et al. Are in the pyramid with the Monks, when the Monk tells the Secretary General that if his consent isn't pure it will kill him, Xiaolian is standing right behind him in shots facing the Secretary General and shots facing the Monk, but in the one shot of the Secretary General as he's being vaporized Xiaolian has vanished, but Xiaolian is still standing in the same spot in the shot facing the Monk. (00:26:10)