The Coconut Effect
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.
This was parodied in the movie Monty Python and the Holy Grail: They didn't actually have horses, just the coconuts. Ironically, the producers actually wanted to use real horses but didn't have the budget and the coconuts did a better job at the whole Rule Of Funny bit.
While audiences have finally outgrown that particular quirk, there are others which persist. The thwpt sound of a gun with silencer (which sounds nothing like an actual silenced pistol); the ''ping'' sound made by a specular reflection; the click of a remote control, so ubiquitous that a recent mobile app phone added clicking sounds to its touch screen; the loud thump of lights turning on or off; or noisy explosions in space.
There are also fistfight noises (the 'whump' of a person getting punched in the face, or the exaggerated smack of a boxing glove) that must be certain way or they won't be believed.
In a medieval setting, whenever a sword is unsheathed, there needs to be a a sound of scraping metal, even if the sheath is made of leather. In sword duels, there is a loud, echoing clash of metal when, in reality, swords just make a small 'tink' sound.
Car and driving noises. "Wildest Police Chases"/"Wildest Security Camera Video"-type programs are big on this. Squealing tires and crunchy crashes are all dubbed in after the fact, especially in the cases featuring security camera footage, which rarely features an audio track.