The pilot episode of The Walking dead opens with Rick at a gas station. I can't remember why he was there (besides the obvious possibility of him getting gas). He encounters a zombie child, she rounds on him and attempts to attack Rick, who is forced to shoot her. This scene has always confused me. Does this even happen, or is it some kind of fictional intro? Second, if this did happen, at what point in time did this occur?
I like to think it sets the mood for the entire series. The world is completely changed and it gives you a taste for just how bad things are if Rick is forced to do what he does.
If you are wanting to know if such a scene ever occurs in the comics, I just checked Volume 1 and saw nothing and don't recall seeing any such scene up through the current issues. I think we are supposed to assume that scene takes place during Rick's trip from Cynthiana, KY to Atlanta, GA.