Dragons, for instance, have the right of safe conduct anywhere in Faërie. A reader may not like to read stories about dragons, she may be morally offended or aesthetically uninterested or simply sick of the subject; but at any rate she will not complain that the author has cheated by bringing in a dragon, because dragons belong in fantasy.
Stories set in the Culture in which Things Went Wrong tended to start with humans losing or forgetting or deliberately leaving behind their terminal. It was a conventional opening, the equivalent of straying off the path in the wild woods in one age, or a car breaking down at night on a lonely road in another.