Continuity is an integral part of the filmmaking process, because it ensures that the action in the film is consistent. For example, if a character is wearing a bandana a certain way in one take of the scene, the bandana needs to be exactly the same when viewed from another angle. This can be problematic when scenes are shot at different times, with breaks between them.
The script supervisor keeps notes on general continuity, and the various departments (wardrobe, hair, makeup, and props) take their own polaroid snapshots to ensure continuity.
The following examples from Lonesome Dove display some of the continuity photos from Property Master Eric Williams, who was responsible for the great mass of props employed in the miniseries-- the saddles, bridles, guns, bows, belts, gear, and bedrolls used by the characters. Williams took hundreds of these snapshots during the filming of Lonesome Dove.