Revealing mistake: Crash mats are visible on the steps, when the Father is thrown through the window and rolls down the stairs.
Revealing mistake: In the scene where Sgt Howie hits McGregor (who is dressed as Punch) with a candlestick, watch closely and you can see that the blow from Howie is aimed into the padded section on the back of the punch costume, rather than at the back of McGregor's head.
Revealing mistake: Right at the end when John is dying look at the texture of his blood. It is way too thin, almost like coloured water.
Revealing mistake: When Jack Hawkins punches through the door it is very apparent that it's made of very thin wood to make it much easier to hit through.
Revealing mistake: The square base securing the knife in Alba's back is visible.