The police knocking on someone's door, and - every time -someone opens the door !
And it's always opened instantly. They never have to wait for the person inside to walk from the kitchen or down the stairs or whatever. They're always standing there right behind the door waiting to open it.
Teachers never doing plenaries and just shouting the homework as the kids all charge for the door after the bell has gone.
That's always bugged me. (And it happened in films and tv shows long before the internet and homework apps existed.) We would never have got away with that when I was at school. If you even started putting your books away, you'd be told to stop it and pay attention. 'The bell is a signal to me that I should think about ending the lesson, not for you to start packing up' was what one teacher used to say.