The Thirty Nine Steps (1978)

Revealing mistake: Towards the beginning of the film, when Hannay has been arrested for the murder of Scudder he is being interviewed by Chief Superintendent Lomas. Hannay removed the bullets from Scudder's pistol whilst he was staying in his apartment, and Lomas is shown putting them on the table. One of the bullet casings has a hole drilled through the center of it, a typical method used to identify inert rounds as being safe to handle.