During a Lecture About Designing Your First Research Study, Your

During a lecture about designing your first research study, your professor (who is also your research advisor) told the class that in general, using a previously validated scale (when there is one available) is preferable to creating all of your own items. However, in a different lecture, your advisor mentioned that just because a scale is validated does not mean it will always be valid. Based on what you know about validity, which statement is correct? In what situations is the first statement better advice? In what situations is the second statement better advice?