An element that is patently unrealistic, but which you have to do anyway because viewers have been so conditioned to expect it that its absence would be even more jarring.
The best example of this is the sound of horse-hooves. From the days of radio, banging two coconut halves together was the standard way to generate the sound effect of horse-hooves. Anyone who has ever actually been around a horse knows that horse-hooves rarely sound anything at all like that, and never sound more than just a very little bit like that. All the same, that sound became so ingrained in the public consciousness that even when it later became possible to insert much more realistic sound effects, the coconut sound effect was still used. The audience wouldn't accept horse hooves making a sound not generated by coconuts.