The Night of the Bleak Island - S4-E21
Continuity mistake: In the opening scene of the storm at sea, the first shot of the large boat shows it to have several windows in the front. When the boat docks, it is evident this is a smaller windowless boat.
The Night of the Bleak Island - S4-E21
Revealing mistake: Towards the end of this episode, West enters a small house. As the door opens, there is a small piece of furniture with drawers in the front and a red and white checker cloth on top. During the fight scene in this room, this chest of drawers is broken to reveal it is a prop that had the drawers painted on the front.