Continuity is always a concern for any filmmaker. When making a motion picture, the filmmaker usually has to shoot a particular scene more than once in order to get the best performance and with different angles in order to get sufficient coverage. When he/she does this, it is crucial to maintain continuity. To maintain continuity is to maintain consistency with the appearance of a set/actors, dialogue, and physical state of props as scenes get shot over and over again. This may seem trivial, but you’d be surprised how many mistakes actually occur in movies. Check out http://www.movie-mistakes.com when you have a spare moment.