Just about anything that says it is going to alter the physical body very drasticly like become a dragon, grow wings,or become half vampire. You can alter people's perception of you with magic like if you don't think people see you as pretty you can do a spell that makes people see you as the most bueatiful guy/girl in the school.
Use common sense, if it sounds far fetched, chances are it is. If it contains any ingredients that you have to ingest, look them up and make sure they will not harm you. Or if something in your gut says it isn't right, then don't do it.