I'm surprised nobody's done this one yet:
The fact that telephone numbers, when read out in movies, must always begin 555.
I understand that this is because there's no genuine 555 area code and so an actual, real, living phone number won't get embossed in film for eternity and the studio won't get its ass sued off but...
The moment any character says 555 it takes you out of the movie and hits your straight between the eyes that you're watching actors in something made up.
A good writer who is making an effort to create a believable world that viewers get immersed in would write their way around this third-wall-shattering trio of numbers.