The theme of children in "adult" films.
I remembered the case when I went to watch "The City of Sins". Then the age census was not strictly followed and some mommy drew a boy of eight years old to the movie. For the first twenty minutes, he was irritated by his wild rust over what was happening on the screen. Then something cried out. But here is the end of the film, include the light - and I long hit the memory of the image of a child with a completely lost expression of the face.
Now, when I come to movies 6+ and I start to boil from the surrounding children - I remember that guy and I immediately feel relieved =)