One of the defining characteristics of the science fiction and fantasy stories is that they take place in a setting different from our everyday world (the “real world”). Even science fiction and fantasy that takes place on Earth must have some key differences that distinguish it from the real world, otherwise it would not be science fiction and fantasy, but would simply be contemporary or historical fiction. However, the fact that science fiction and fantasy stories are set in such an imaginary world requires one key concession from the reader or audience: suspension of disbelief.
Suspension of disbelief is simply the willingness of an audience or reader to temporarily accept what is clearly untrue in reality for truth in a given fictional setting. For example, while dragons and other magical creatures do not exist in the real world, an audience or reader must “suspend their disbelief” and accept that dragons and magical creatures do exist in the context of a given fantasy setting. Even so, there are limits to how far an audience or reader will go in suspending their disbelief. While they may accept that dragons exist in a certain fantasy setting, they will not accept a dragon that in one scene is red and then in another scene is blue (unless it has been established that dragons can change color). In other words, suspension of disbelief cannot be used as a convenient “out” to cover-up plot holes, contradictions, logical fallacies, or shoddy historical research. In order to facilitate suspension of disbelief, the work must at least be internally consistent.
Without internal consistency, a work of science fiction or fantasy will quickly dissolve into absurdity and chaos. All writers strive to remain internally consistent within their own work–even parodies need to be consistent in their satirical mockery. But mistakes do happen, and when a reader stumbles upon such an error in consistency, they are rudely jarred from their suspended disbelief into the sudden realization that not only what they are reading is make-believe, but that the author is sloppy as well. There is simply no excuse for such embarrassing mistakes because they can be avoided easily enough if an author simply takes the time to build his or her world.