Continuity

Continuity is the consistency of the characteristics of people, the plot, places, objects and events seen by the viewer. If continuity is not monitored and carried out during filming then people watching could spot the mistake and it may distract them from the story and what's going on. If someone is not there to spot the mistakes made during filming then the scene may have to be re-filmed, and this creates the problem of making sure that every single factor (appearance, time of day etc.) is the same as it will otherwise not flow correctly and look out of place.

Here is a film which highlights the mistakes than can be made through continuity