If someone gets injured, and it is proven that what caused the injury is the result of an incorrect movement no longer supported by current research or a movement not indicated for a particular client, than the risk is a lawsuit. Ugh. Who wants that?
Be mindful of why you are prescribing a certain exercise to a certain client. There are also some exercises that are too complicated to be done in a class setting – they need more one on one feedback to be done correctly. So stay on top of the literature and be able to think critically about the volume of information that is “hot” in the industry.