Cheating is a term set designers use when they design sets that are seperate shooting entities, yet are filmed and portrayed as the same location. There are several reasons a set designer may cheat a set. A studio set may only have the stage space to build a lower level of a house, requiring a separate set to be built that would play as the upper level. Usually in this case, the cheat is in the stairwell; it gets built twice. Sometimes the film gets shot, only to need additional scenes to be shot at a location that can no longer be used. If the scene was a set that has been torn down, a record of the construction plans, paint colours, lighting and set dressing is crucial to recreate some kind of continuity. Sometimes a location is perfect, except for a particular scripted area, which needs to be cheated.
Cheating is fun! Its like a big game of 'spot-the-difference'.