Doctor Who (1963) - 3 mistakes in series 20
All of series 20
Other: Episode - The Five Doctors. The Cyber Leader is supposed to follow The Master's footsteps exactly in order to get across the chessboard. He doesn't. This cannot be classed as a character mistake because the Cyber Leader knew what was involved. I believe this mistake was caused by the actor not being able to see the chessboard through his helmet.