The girl who played Sarah Louise quit at age 12 because she was being teased at school and because strangers were talking to her when she was out and about. How weird would you have to be as an adult to approach a child, albeit a famous one, in the street?! It’s probably odd enough being approached at 16+ but it must be really unnerving for a kid.
Sarah Louise was originally played jointly by that actress and her twin Leah, but Leah died. Lynne Petrie wrote about it in her autobiography and said that the cast and particularly Helen Worth were extremely upset.