If it is an essay or something it's harder to tell whether you should have been awarded the marks or not.
If it was multiple choice or something I would defo tell them, particularly because we go over it rigorously and it could potentially get you into a lot of trouble!
However, despite saying that (and not having any marking issues in year 12, although some in previous years that I brought up) if the marks were the disparity between a rank or between a report grade... I don't know if I would have the moral fibre to tell them!