Revealing mistake: One of the burning zombies at the very end of the film is wearing the most terrible and obvious fire-proof mask and hair-piece, made all the more noticeable by being used so prominently in front of the camera several times.
Revealing mistake: When Sir James breaks into the house of Squire Hamilton, we are shown a long shot of the tin mine underground with the zombies being whipped by the Squire's men. Watch the back of the shot - several blows from one man hit a couple of the wooden props supposedly supporting the roof, which obviously wobble about and show they are just for decoration rather than any practical purpose.
Revealing mistake: The zombie of John Martinus throws the lifeless body of Alice from the tin mine pit head onto the ground. When Sylvia examines the body after, Alice very obviously blinks. Visible at normal speed, but easier to see slowed down.