Continuity

'Continuity is a film term that suggest that a series of shots should be physically continuous, as if the camera simply changed angles in the course of a single event. For instance, if in one shot a beer glass is empty, it should not be full in the next shot. Live coverage of a sporting event would be an example of footage that is very continuous. Since the live operators are cutting from one live feed to another, the physical action of the shots matches very closely. Many people regard inconsistencies in continuity as mistakes, and often the editor is blamed. In film, however, continuity is very nearly last on a film editor's list of important things to maintain.

Technically, continuity is the responsibility of the script supervisor and film director, who are together responsible for preserving continuity and preventing errors from take to take and shot to shot. The script supervisor, who sits next to the director during shooting, keeps the physical continuity of the edit in mind as shots are set up. He is the editor's watchman. If shots are taken out of sequence, as is often the case, he will be alert to make sure that beer glass is in the appropriate state. The editor utilizes the script supervisor's notes during post-production to log and keep track of the vast amounts of footage and takes that a director might shoot.

Editors can choose between emotional and storytelling aspects of any given film over continuity- something that is much more abstract and harder to judge. (Which is why films often take much longer to edit than to shoot.) Emotional continuity, and the clarity of storytelling, can take precedence over "technicalities". In fact, very often something that is physically discontinuous will be completely unnoticeable if the emotional rhythm of the scene "feels" right. If you were to slow down scenes from many of your favorite movies, you could easily find many minuscule physical differences from one cut to the next, which are completely hidden by the course of the emotional events.

However, if a continuity error is glaring enough (as in the case of the beer glass), and the edit is emotionally necessary, it is increasingly common to order a visual effect to fix the problem. Such an effect is not "cheating" or unnecessary: as a rule, anything that distracts from the storytelling is worthy of elimination.

A good example of a continuity error is in the film Braveheart with Mel Gibson. In one of the battle scenes you see William Wallace (Mel Gibson) and his army of Scottish rebels charging into battle with the English. At one moment, you see him with no weapon. Then you see him with his claymore in hand. Then again he has no weapon. Then a pick axe. And when he finally closes in on the enemy, you see him draw his claymore from his back. This often goes unnoticed by audiences and it does not cause any real problems. The whole idea of the scene is to show the rebels fiercely charging into battle, and these errors do not actually interfere with that.'

- http://en.wikipedia.org/wiki/Film_editing